The production of thin or thin films of metals or ceramics by chemical vapor deposition has often been achieved by the use of halide gas precursors. In certain cases, this choice was made purely for reasons of simplicity: gas cylinder, gas species already used in another field, etc. Experience has subsequently shown, however, that this choice can give rise to significant changes in the nature and proportions of deposited phases. These are highly dependent upon: the value of the oxidiser: reducer ratio in the gas phase, the degree of metal oxidation in the halide considered, and possible competition between two reducing agents designed to reduce the halide. These factors, among others, strongly influence the thermochemistry of the deposition reaction. Their roles must therefore be clearly understood, interpreted and predicted by the thermochemical analysis. Based on examples relating to silicide, nitride and boride deposits, an attempt will be made to determine sensitive parameters and deduce selection criteria.