Here, we study the decomposition of NH3 at CVD conditions, with careful consideration of the decomposition kinetics, by kinetic modelling using reaction mechanisms and reaction rate data from the literature. We find that the decomposition of NH3 is very slow and the decomposition products will not reach their highest partial pressures during the typical residence time in the CVD reactor. We argue that this slow decomposition kinetics of NH3 in CVD explains the need for the very high NH3 to metal precursor ratios needed.