It is well known that the properties of reactively sputtered thin films of tantalum nitride are dependent upon the partial pressure of N2 in the sputtering gas. In this study, a Ta target was rf diode sputtered using various Ar/N2 gas mixtures. Glow discharge spectrometry was used to monitor the positive ions present in the sputtering plasma. Electrical and x-ray diffraction measurements on the resulting films related their crystallography and resistivity to the composition of the plasma, in particular to the relative number of Ta and TaN ions. The results are discussed in terms of the change in the target surface with increasing N2 partial pressure.