In this study, the interaction between tin(II), thiosemicarbazide and selenium(IV) was examined polarographically. It was seen that in acidic medium, there is a reaction between selenium(IV) - tin(II) and selenium(IV) - thiosemicarbazide, but there is no reaction between thiosemicarbazide tin(II). It was observed that, when Se(IV) is added to Sn(II)-thiosemicarbazide mixture, there is a sharp decrease at the reduction peak of Sn(II), but there is no decrease at the reduction peak of thiosemicarbazide. As the amount of selenite increases, it was observed that after the disappearance of reduction peak of Sn(II), decrease at the reduction peak of thiosemicarbazide starts, and also, the reduction peak of selenium (IV) which appears around -550 mV disappears at the new polarograms that are obtained by just waiting, (without making any process) and during this process thiosemicarbazide peak decreases more. As a result, it was found that there is a more rapid and having a priority reaction between Sn(II) - Se(IV), than thiosemicarbazide - Se(IV). It was also found that thiosemicarbazide - Se(IV) reaction appears after all Sn(II) in the medium is consumed. The decrease at the thiosemicarbazide peak, after waiting a certain time, proved that this reaction is slow.