A high-vacuum plasma-assisted chemical-vapor deposition system was used to systematically study ZnO:N thin film synthesis. Nitrogen doping was achieved by mixing either N2O or N2 with O2 in a high-density inductively coupled plasma (ICP) source. In situ diagnostics showed that the ICP composition was predominantly a function of the elemental oxygen to nitrogen ratio, and relatively insensitive to the choice of N2 or N2O as the molecular precursor. Nitrogen incorporation was measured by both x-ray photoelectron spectroscopy and secondary ion mass spectrometry and was found to increase monotonically with both N2O and N2 addition. Nitrogen doping was correlated with systematic shifts in the lattice spacing, electrical conductivity, and optical absorption. Quantitative comparisons between film properties and gas composition suggest that atomic nitrogen is the primary precursor for doping in this system.