In this work, the influence of the ambient gas (He, Ne, N-2, air, Ar and Kr) on the mechanisms of particle formation and deposition in laser micromachining of Al2O3-TiC ceramics using pulsed 193 and 248 nm radiation was investigated. Two kinds of particles redeposited around the processed area could be identified: ultrafine particles, of a few tens of nanometers (debris), which aggregate upon redeposition to form a continuous film; and larger particles, with a diameter of 0.5-8 mum (particulates), which remained individualised. The behaviour of debris follows blast wave theory predictions, indicating that this type of particles results from collisions behind the ablation plume front. The particulates are formed above a threshold laser fluence, which decreases with increasing ambient gas atomic/molecular weight and pressure. This threshold is lower for 193 than for 248 nm radiation, but shows the same dependence on the atmosphere for both wavelengths. Experimental evidence and theoretical analysis suggest that the particulates result from ejection of droplets from growing protuberances on the irradiated surface, due to the recoil pressure produced by the plume. (C) 2003 Elsevier Science B.V. All rights reserved.