The elemental composition and chemical states of a series of Mo nitride thin films have been investigated using x-ray photoelectron spectroscopy (XPS). The chemical composition and stoichiometry of the films were also tracked as a function of the preparative parameters: the nitrogen ion dose, ion accelerating energy and substrate temperature. At the lowest ion dose, the Mo concentration increased to a depth of ~300 A. The nitrogen concentration in the film decreased with increasing the ion energy. An increase in the ion energy to 100 and 200 kV led to a lower N/Mo ratio as compared to that at 50 kV. The magnitude of the binding energy separation at room temperature was smaller than that at higher temperatures. This difference might be due to the presence of B1-MoN at higher temperatures and the existence of δ-MoN at lower temperatures.