We report on the control of nanocrystal sizes in US nanocrystalline films prepared by ammonia-free chemical bath deposition technique. We studied the effect of deposition duration. bath temperature during deposition. and post-preparation heat treatment. Nanocrystals (NCs) with radii from 2.6 nm to more than 10 nm were prepared and characterised by transmission electron microscopy, X-ray diffraction, and optical photoluminescence and absorption spectroscopy. We observed cubic to hexagonal phase transition for large NCs prepared by the heat treatment. (C) 2002 Published by Elsevier Science B.V.