N-type GaAs substrates have been exposed to zinc atmospheres at 600-800 deg.C and focused light from a Kr-ion laser at 647.1 nm. Etch pits are generated at optical densities of 0.2-10 KW/cm2 and exposure times of 5-60 minutes. They are characterised by a smooth profile with typical diameters of 50-80 μm and depths of 0.1-50 μm. The etch depth depends strongly on the laser power and on the zinc and arsenic vapor concentrations. The onset of etching is characterized by an increase in surface reflectivity of up to 10 per cent which is probably due to the formation of a thin liquid metal film. The growth of this film is initiated by optically generated holes at the semiconductor surface. The film is not saturated and dissolves GaAs from the substrate. In addition to etching, thermal diffusion of zinc can be observed in samples subjected to long exposure times or high temperatures