Monte Carlo simulation is used to study the role of chemical reactions in the gas flow of particles laser desorbed from the target into a vacuum. The influence of recombination and dissociation processes on the properties of the gas flow is considered. It was found that chemical reactions have a significant effect on the composition of the desorption jet and on the angular and mean energy distributions of the desorbed particles. The study of these phenomena is of a particular interest for the understanding of the process of thin film deposition by pulsed laser ablation.