A combination of high pressure - high temperature synthesis and laser-heated diamond anvil cell experiments, using synchrotron X-ray diffraction has been used to investigate the phases in the Mo2N-MoN system under these conditions. Under conditions of high nitrogen activity, -MoN appears to be a limiting phase i.e. there is no evidence for formation of new compounds such as Mo3N4 or Mo3N5, that are encountered in the Zr-N, Hf-N or Ta-N systems. When -Mo2N is heated or prepared under conditions of high N2 activity, -MoN is formed. The results indicate (a ) that there is no stable cubic stoichiometric gamma-MoN phase formed even under conditions of high pressure - high temperature and high N2 activity, and (b ) that no solid solution is formed between gamma-Mo2N and delta-MoN.