Nanosecond nondegenerate optical pump-probe experiments have been performed on InGaN thin films and InGaN/GaN multiple quantum wells. Bleaching of absorption of the localized band tail states was observed with increasing excitation density (I-exc) of the pump pulse. The dynamics of the bleaching was found to depend on the localization depth of the band tail states and on I-exc. With high I-exc, large blueshifts in the spontaneous emission luminescence peaks were also observed, the magnitude of which was again found to depend on the localization depth of the band tail states. Stimulated emission is observed from the samples with increasing I-exc and correlates with significant changes in the behavior of the absorption bleaching. The observed bleaching dynamics of the band tail states are well explained by considering the effective lifetime of the band tail states as measured by time-resolved photoluminescence experiments. (C) 1998 American Institute of Physics. [S0003-6951(98)04439-8].