GaNvAs1-x films were grown on GaAs substrates by metalorganic vapor phase epitaxy (MOVPE) using trimethylgallium (TMGa), tertiarybutylarsine (TBAs) and 1,1-dimethylhydrazine (DMHy). Nitrogen incorporation in the films was determined by high-resolution X-ray diffraction measurements. The measured nitrogen contents were significantly higher than the solubility limits predicted by delta lattice parameter (DLP) or valence force field (VFF) models for the bulk alloy. A thermodynamic model for mixing on the anion sublattice was used to describe the solid composition as a function of the gas phase composition. The analysis clearly shows that the solid material is produced through reactions involving atomic As and N (or their radicals), rather than tetramers or dimers. We determine an effective interaction parameter of the regular solution model, Omega (eff) = 3080 +/- 530cal/mol, that is significantly smaller than predicted by the DLP or VFF schemes. The discrepancy is explained through considerations of the epitaxial strain which reduces the unfavorable enthalpy of mixing. Our results suggest that the optimum control over N-incorporation is achieved with a TBAs/TMGa ratio which is as close to unity as possible without compromising material quality.