The reduction of selenium(IV) in aqueous solution at pH 6.1 is described employing optical and conductometric pulse radiolysis methods. The reduction of selenium dioxide by hydrated electrons yields SeO₂⁻ with a rate constant of 1.7×10⁹dm3 mol⁻¹s⁻¹. On the 1 ms time-scale, this reduction is followed by a number of processes yielding Se₂O⁴₂^-HSe₂O₃⁵⁻ and H₂Se₄O₆⁻¹⁰ as intermediates with the rate constants 2k₂=2×10⁹,k₃=1×10⁹ and 2k₄=1.6×10⁸dm³mol⁻¹s⁻¹. The extinction coefficients of the intermediates have been determined by simulating the kinetic traces at different wavelengths, and based on those values the respective absorption spectra were constructed. If the solution is irradiated by several pulses of high-energy electrons or with ⁶⁰Co γ -rays, small selenium clusters are produced exhibiting a broad absorption with a maximum at 270nm. These results suggest that the final product of the reduction of Se(IV) by hydrated electron is the selenium aggregate. The selenium clusters are stable for some months in the presence of air and are characterized using transmission electron m icroscope with a diameter ranging between 50 and 120 nm.