Chemical vapor deposition of Ta on the polyimide pyromellitic dianhydride oxydianiline was attempted using TaF5 as the precursor. The deposition temperature was 350°C. Photoelectron spectroscopy measurements show that Ta is bound in a high oxidation state. Our data show strong evidence that initially the ternary compound TaOF3 is formed on polyimide in a first step. The carbonyl group is the main reactive site upon TaF5 exposure. It has to be noted that no passivation of the surface takes place and that there is no limit for film thickness. At very high doses, however, the film concentration at the surface is suggestive for the stable compound TaO 2F. This observation is explained in terms of the formation of a metastable TaOF3 compound in the first stages of film growth and a phase transformation to TaO2F at larger thicknesses. Annealing experiments to 500°C suggest the transformation to the oxide Ta 2O5 that is strongly related to the almost complete loss of fluorine from the film upon heating.