The uranyl-selenium(IV) interaction was studied by time-resolved laser-induced fluorescence spectroscopy in aqueous perchloric acid solutions containing 1 x 10(-4) M U(VI) and Se(IV) at different concentrations (from 0 up to 0.3 M) at pH 1 and 2. The quenching of uranyl fluorescence is observed. HSeO(3)(-) is demonstrated to be responsible for the fluorescence quenching. Stern-Volmer analysis gives the dynamic quenching rate constant value (k(q)) (5.0 +/- 0.4) x 10(9) L M(-1) s(-1) at ionic strength (mu) 0.05 M. The bimolecular excited-state process is shown to be diffusion-controlled as k(q) is practically identical to the diffusion rate constant as calculated for uranyl and hydroselenite charged species. With an increase of HSeO(3)(-) concentration, static quenching occurs in addition to the dynamic quenching. The hypothesis of the formation of a weak fluorescent ground state complex between uranyl and HSeO(3)(-) is supported. The logarithm of the stability constant value (beta) is found to be 3.35 +/- 0.12 (mu= 0.05 M) and 3.32 +/- 0.15 (mu = 0.2 M). These constants are confirmed by UV-Vis spectrophotometry. The modelling factor analysis of absorption spectra gives logbeta = 3.35 +/- 0.17 (mu = 0.035 M). Extrapolation to mu = 0 by the specific ion interaction theory (SIT) gives logbeta degrees = 3.62 +/- 0.15 at 20 degrees C.