The role of vocabulary in foreign language learning is crucially important. Along with other techniques, using glosses is a popular vocabulary technique in language learning. The present study focused on text based glosses which is a type of various glosses. This study investigated L1 and L2 gloss effects on incidental vocabulary learning through reading. 44 intermediate level prep EFL learners participated in the study which was conducted at a private University in Konya, Turkey. The students were divided into two groups: L1 gloss and L2 gloss. The students were provided with different texts and a gloss type specially designed for their own group (either L1 or L2 gloss). The students took a pre-test, a post-test, and a delayed post-test. The delayed post-test was taken 3 weeks after the post-test. The scores were gathered from these tests and the data was analyzed using independent samples and paired (dependent) samples T-tests. The students’ scores from production and recognition tests were compared. The results of many comparisons revealed significant differences in favor of L1 gloss group. However, no significant difference was found for L2 gloss group’s scores in paired samples T-test analysis. Findings suggest that using glosses for incidental vocabulary learning is useful for students.One should know the basic tenets of vocabulary acquisition techniques to be a successful foreign language learner. As glosses are some of those techniques which help students to promote their vocabulary repertoire, using L1 and L2 glosses in classroom environment will help them to be more proficient language learners. For Ko (2012), glosses are modified form of input and adding these kinds of information such as synonyms, definitions and extra descriptions help learners while reading texts which have insufficient cues. Different glossing techniques can be used such as L1 versus L2 glosses, computerized versus paperbased glosses, textual versus multimedia glosses, or single versus multiple choice glosses. The present study focused specifically on text based glosses because Farvardin and Biria (2011) claimed that students preferred definitional glosses over other types of glosses. The pioneering figures on glosses Holey and King (1971), Watanabe (1997), Nation (2001) asserted that these glossing techniques will increase students chance to learn target vocabulary in reading texts. On the other hand, Mondria (2003) and Johnson (1982) attested that with their studies on the negative effects of glosses in terms of interrupting reading flow and decreasing student interaction with their learning process. Although there are various studies indicating that the positive effects of glossing techniques in vocabulary learning, one can see that there are controversial approaches using both L1 and L2 glosses in literature. Glosses have positive effects on the learner comprehensionof texts, have them derive the meaning of words from glosses, make learners autonomous, help to increase their L2 input processing, and also glosses decrease the intervening effect of looking up a dictionary while reading Nation (2001), Lenders (2008). In the light of all this information, the purpose of the study here is to investigate L1 and L2 glosses’ effects on incidental vocabulary learning by asking the following research questions: 1. Is there a difference between L1 and L2 gloss groups’ immediate post-tests? 2. Is there a difference between L1 and L2 gloss groups’ delayed post-tests? 3. Is there a difference between immediate post-test and delayed post-tests of L1 gloss group? 4. Is there a difference between immediate post-test and delayed post-tests of L2 gloss group? A total of 44 students from two classes at the same university’s preparatory program participated in the study. The students’ level was intermediate. They were learning English as a foreign language at a private university’s preparatory program. The students were divided into two experimental groups. One group used L1 glosses while the other used L2 glosses with reading materials. The study employed T-test to analyze the data. The independent variable was the gloss type used for reading materials. The dependent variable was students’ scores measured by the immediate and delayed vocabulary tests, and each test had both production and word-recognition. In the treatment part of this research, firstly the students were provided with the pre-test to check if they know the words on the list given or not. So that the target words were chosen. During the treatment session, the students continued reading their texts and doing reading activities as usual. However, this time the students were given glosses at the bottom of the page on which the reading passages were. Every day, the students read and did the reading activities of just on text. After reading and doing the exercises, on the same day the students were given two kinds of tests (vocabulary production test and word recognition test) which were immediate post-tests. Three weeks later delayed post-tests were given to the students which has the same format of immediate posttests but with a different order of the same questions. After all the tests were conducted, the means of the groups for each test were compared with independent samples T-test. Also, the difference between immediate and delayed post-tests was compared with paired samples (dependent) t-test for L1 and L2 gloss groups. The results of the pre-test showed that 21 words out of total 39 words on the list were marked as unknown by the students and were determined as target words for the current research. The first research question compared L1 and L2 gloss groups’ immediate post-test results. The results showed a significant difference between two gloss groups for immediate production and immediate recognition tests. This result corresponded with those of previous studies (Ahangari and Dogolsara, 2015; Bayraktar, 2008; Spahiu, 2000) showing that L1 glosses