Several models of UV laser-induced darkening of titanium-oxide wet gels are considered, which fit well to the recently obtained experimental data on the darkening kinetics. They reveal a high initial quantum yield of the light-induced charge separation and the existence of a 10% limitation in the number density of the trapped electrons, as compared to the number density of possible traps. This limitation is attributed to the internal photo-effect, the recombination of trapped electrons with the fast holes and the interaction of newly created free electrons with the already trapped electrons. (c) 2005 Elsevier B.V. All rights reserved.