Elements of arsenic and selenium have been the centres of attention for researchers regarding their important roles in metabolism. Selenium can be both essential and toxic depending on its concentration level, while arsenic is a toxic element. Both elements are also in focus of interest for their speciation analysis. In literature, there are studies on their antagonistic and/or synergic interactions. In this project, an analytical method based on enzymatic extraction followed by determination using HPLC-ICP-MS was developed for the simultaneous extraction and speciation of arsenic and selenium. Developed methodology was applied to green onion plants that were grown in Hoagland nutrient solution spiked with arsenic and/or selenium in order to investigate possible arsenic-selenium interactions. In the samples fed by As(III) or As(V) spiked solution, most of the arsenic was absorbed by the plant and deposited in the root as As(III) whereas transport to bulb and leaves was limited. On the other hand, selenium absorption was lower compared to arsenic; when plants were supplemented with Se(IV), most of the selenium was found in root and leaves in the form of selenomethionine and selenomethyl selenocysteine. Selenium was preserved as Se(VI) and only very small percentage was converted to organoselenium compounds in plants supplemented with Se(VI). Analysis of feed solutions revealed that conversion between As(III)-As(V) and Se(IV)-Se(VI) couples in feed solution was insignificant. This result is a significant contribution to the novelty of the present research findings compared to the similar studies in literature. Synergic and antagonistic relations were observed and determined in a detailed manner in case of simultaneous supplementation of arsenic and selenium. It is known from the literature that different interaction mechanisms were present for different plants. Results given in this study are valid only for green onion plants grown under specific conditions and should not be generalized for other plants.