Properties of semiconductor nanocrystalline thin films are significantly influenced by quality of the nanocrystalline surface. We report on a strong effect of ammonia treatment on optical properties of CdSe nanocrystalline thin films prepared by chemical bath deposition. The treatment affects strongly photoexcited carrier dynamics, as measured by ultrafast absorption and photoluminescence laser spectroscopy, which reveals a crucial role of surface states in involved processes. The experimental data are interpreted in terms of ammonia passivation of the nanocrystal surface states. (c) 2006 Elsevier B.V. All rights reserved.