The uranyl-selenium(IV) and uranyl-selenium(VI) interactions were studied by CE in aqueous acid solutions, containing U(VI) and Se(IV) or Se(VI) at different concentrations, at pH 1.5, 2.0 and 2.5. The method proposed in this paper allows one with the use of CE data on metal ion mobilities at different pHs to establish the ligand species interacting with metal ion and complex species formed. In the case of Se(VI) a selenate, as demonstrated, interacts with uranyl ions, in the case of Se(IV) this is a hydroselenite. It was also shown that the equilibria for the U(VI)-Se(VI) and U(VI)-Se(IV) systems can be established from CE data. The formation of UO2SeO4, UO2(SeO4)math image, UO2HSeOmath image and UO2(HSeO3)2 species is demonstrated. The stability constant values were measured at different ionic strengths (from 0.02 to 0.2 mol/L). The logarithms of the stability constant values (β°) extrapolated to ionic strength 0 by the specific ion interaction theory (SIT) are found to be log β°1=2.93±0.06 for UO2SeO4 formation, log β°2=4.030.18 for UO2(SeO4)math image formation, log β°1=3.270.15 for UO2HSeOmath image formation and log β°2=5.510.11 for UO2(HSeO3)2 at 25°C. The results for the first constant values for each of systems are consistent with the published values. For UO2(SeO4)math image formation, a new constant stability value is given. The existence of UO2(HSeO3)2 complex species is demonstrated and its constant stability value is given for the first time.