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By using picosecond or nanosecond laser photolysis to excite colloidal (diameter 120 Á) particles of Ti02 in aqueous solution, we have monitored the dynamics of charge carrier trapping and recombination in this semiconductorand interpret the results by a stochastic kinetic model. While the absorption spectrum of the trapped electron appears within the leading edge of the 30-ps laser pulse, the trapping of the hole is a much slowerprocess requiring on the average 250 ns. At high electron-hole pair concentration in the particles, their recombination follows a second order law, the rate coefficient being (3.2±1.4) X 10" 11 cm3 s" 1. Intraparticle recombination becomes first order at very low charge carrier occupancy, the lifetime of a single electron-hole pair in a 120 Á sized TiOz particle being (30±15) ns. Under these conditions, hole trapping, presumably by surface hydroxyl groups, competes with recombinationand …