The causes of nonadherence of chemically vapor-deposited coatings are considered. The absence of substrate attack by the gases present in the system is shown to be a necessary criterion, in addition to those usually cited, for coating adherence. The importance of this criterion is illustrated for a number of coating experiments from the literature. The use of standard free-energy data for predicting substrate attack is also illustrated. Cases in which standard free-energy data alone are not sufficient are also considered and illustrated for the adherence of tungsten coatings on tantalum substrates.