In this work colloidal quantum dots (QDs) of GaAs and CdS semiconductors have been formed by laser ablation in the liquid media. The pulsed passive mode-locked Nd:YAG laser at 1064 nm wavelength with pulse duration tau(imp) = 33 ps and energy 30 mJ was used. The luminescence of the colloidal QDs was excited by irradiation at 355 nm, the third harmonic of the Nd:YAG laser. The optical absorption and the photoluminescence spectra of the GaAs and US colloidal QDs have been investigated. The large blue shift of the photoluminescence, connected to size effects, was evaluated. The location of the maximum of luminescence spectra at the wavelengths 405 nm (CdS) and 420 nm (GaAs) give calculated sizes of QDs of 2-3 nm. (c) 2005 Elsevier B.V. All rights reserved.