The ion composition of the plume produced in the laser ablation of silicon monoxide (SiO), at the laser wavelengths of 532, 540, 308, 248 and 193 nm, is studied by time-of-flight mass spectroscopy (TOF-MS). The mass spectra is found to be dependent on laser wavelength. The most distinctive difference is that Si-n(+) and SinOn+ clusters, that are formed upon ablation at the longer laser wavelengths, are absent in the ablation at the short laser wavelengths. Ion velocities in the plume are measured. At 248 nm the ion velocities are strongly dependent on mass size whereas only a weak dependence is observed at 308 nm. The differences are discussed in terms of two different ablation mechanisms operating at the shorter and longer laser wavelengths. (C) 2002 Elsevier Science B.V. All rights reserved.