We have synthesized a range of Mo–Nb nitrides at 2.5 GPa and T = 1600 °C or 2200 °C using high pressure–high temperature techniques. Following syntheses at 1600 °C Mo-rich compositions are hexagonal and Nb-rich phases are cubic, with a narrow two-phase region indicating the presence of a solvus. The maximum Mo content in the cubic phase derived from δ-NbN is 40–44 mol%, and the maximum Nb content in the hexagonal δ-MoN phase is 46–47 mol%. There was little variation in the superconducting transition temperature Tc for hexagonal δ-(Mo,Nb)N samples produced in the study. Previous studies showed maximum Tc values of 12–15 K for pure δ-MoN as a function of N-site ordering in high-P,T experiments. Here we recorded Tc = 14 K for the limiting Mo-rich composition prepared in the study. We observed Tc = 11.5–16 K for cubic δ-NbN depending on high-P,T synthesis and annealing conditions. This value falls to 5 K for the cubic Mo0.37Nb0.63N0.98 solid solution phase. Some high-P,T synthesis or annealing experiments were carried out at T = 2200 °C. At this temperature δ-MoN decomposes to produce γ-MoN0.54. A minor phase in this sample achieves Tc = 9.5 K. A new superconducting hexagonal oxynitride MoN0.74O0.38 with Tc = 16 K was also produced during this study.