US films were prepared on a glass substrate and their growth was analyzed as a function of the supersaturation ratio in dilute solutions. Nanocrystalline US with a hexagonal (wurtzite) structure is present in the US films and aggregated into colloidal particles. The particle size increases from 117 nm to 357 nm when the relative supersaturation ratio (S,) increases from 0.4 to 1.0, but particle size decreases to 149 nm and the absorption edge blue shifts from 488 nm to 456 nm when S, increases further to 4.0. The evolution of the US film is explained on the basis of the proposed growth kinetic model. (C) 2007 Elsevier B.V All rights reserved.