The formation of thin films in the conventional thermally activated chemical vapor deposition process is based on heterogeneous reactions involved in surface nucleation and growth. As the influence of homogeneous reactions has often been underestimated, this paper addresses the various cases that may be encountered in chemical vapor deposition, from standard deposition conditions to highly reactive gas phases. First, the influences of the various species that are formed during gas-phase transport are illustrated by simulation models. The case of the highly reactive gas phase leading to competing mechanisms between homogeneous and heterogeneous nucleation is then discussed. Finally, the mechanisms responsible for powder formation at high temperature and high supersaturation are presented.