The kinetics of the 420 nm luminescence emitted from H2O and D2O polycrystalline Ih ices have been studied over the 77 to 162 K temperature range. In the case of both H2O and D2O ices, it was found that the luminescence rise and decay curves consisted of two luminescence components, and superimposing two first-order curves with different rate constants gave the best fit to the decay and rise curves. The mean lifetimes of the two luminescence components were 1.08 +/- 0.03 s and 2.47 +/- 0.03 s. The rate constants were found to have negligible temperature dependences, which led to activation energies well below those obtained for either activation-limited processes or even diffusion-limited processes. Furthermore, it was found that the luminescence kinetics were not affected by isotopic substitution of D for H in the ice lattice. These observations suggest that the rate-determining step in the mechanism for the production of the luminescence is a slow (probably spin-forbidden) electronic transition that can occur at two different rates due to the presence of two different types of trapping sites in the ice lattice. A possible candidate for the electronic transition is the (4)Sigma(-) -> X-2 Pi transition of excited OH. radicals and not the previously suggested and ubiquitous A2 Sigma(+) -> X-2 Pi transition of this species.