Metal films grown by photolysis of Cr, Mo, and W hexacarbonyls are known to contain large amounts of carbon and oxygen, suggesting incomplete removal of CO from the precursor as well as possible reactions with other sources of carbon and oxygen. In order to identify microscopic processes responsible for film composition, a systematic study of thin films photochemically deposited by continuous, low-power 257-nm light from Cr, Mo, and W hexacarbonyls has been carried out. Since photodissociation of the precursor can occur on the surface as well as in the gas phase, experimental conditions have been chosen such that surface reaction kinetics are rate limiting. The experiments show that background gases during deposition and exposure of newly deposited films to air both result in significant oxidation of the films through their entire thickness, as determined by scanning Auger microscopy and sputter depth profiling. Films deposited in ultrahigh vacuum and analyzed without exposure to air have compositions consistent with the stoichiometries CrCO, MoC2O, and WC<1O<1. Spatially resolved analyses of film compositions has allowed a distinction to be made between photochemical and spontaneous reactions at the surface of the growing film. The results are discussed in terms of studies of photolysis of the metal hexacarbonyls and dissociative chemisorption of CO on clean crystalline metal surfaces. © 1990 American Chemical Society.