Abstract Both dynamic and steady-state luminescence methods have been applied to study the emission of aqueous solutions of uranyl ions (UO 2 2+) to attempt to reconcile the various models currently presented for this system. From flash photolysis measurements, it is shown that the emitting ∗UO 2 2+ state is fully formed within the lifetime of a 30 ps laser pulse. Emission spectra are presented at room temperature and in a low temperature glass for solutions at low pH, and assigned to luminescence from ∗[UO 2(H 2O) 5] 2+. On increasing pH (1.6<pH<3) broadening of the emission bands, changes in emission quantum yields, and red shifting of maxima are observed. There are no changes in ground state absorption spectra in this region. The changes are accompanied by the onset of biexponential kinetics in the luminescence decay, and are attributed to acid-base dissociation in the excited state. In agreement with previous reports, changes observed at higher pH and uranyl concentration are assigned to the onset of hydrolysis of uranyl in the ground state.