Second-order nonlinear optical properties of US colloids with thiourea as the surface-capping agent were experimentally studied in water by hyper-Rayleigh scattering (HRS) technique. The particles crystallized in cubic zinc blende structure with mean diameter in 5 urn. The UV-VIS absorption spectra indicate that the nanoparticles have a negligible absorption at incident light of 1064 nm and frequency-doubled light of 532 nm. The intensity of HRS signals corresponding to pH variance exhibited a maximum at pH 4.1. The result was explained by progressing protonation of different groups on the particle surface in solution, based on the two-level model. (C) 2002 Elsevier Science B.V. All rights reserved.