High-temperature X-ray diffractometry in a pressure-controlled high-purity nitrogen atmosphere was used to investigate the phase transition γ-Nb4N3±x→δ-NbN1−x. The transition occurs at 1135±5°C (for a sample at the nitrogen-rich phase boundary of the γ-Nb4N3±x phase at [N][Nb]≈0.80 and is fully reversible. The separation of the (200) and (002) diffraction lines of γ-Nb4N±x smoothly decreases upon increasing temperature until the pattern becomes that of the fcc δ-NbN1−x indicative of a quasi-continuous transformation. Nitridation/denitridation cycles at higher temperatures than the transition temperature yield the reaction β-Nb2N+N2→δ-NbN1−x without γ-Nb4N3±x as an intermediate phase representative for the absence of a γ-Nb4N3±x+δ-NbN1−x two-phase region.