The temperature effect of the ammonium halogenide mineralizers NH 4X (X = Cl, Br, I) on the phase stability of GaN synthesized under supercritical ammonothermal conditions in the temperature range 360-550 °C has been investigated. Hexagonal GaN (h-GaN) and cubic GaN (c-GaN) were crystallized. Oxygen impurities force the formation of gallium oxide at low temperatures. The tendency to form c-GaN increases from X = Cl to Br to I. Decreasing temperature supports this trend. Single-phase h-GaN can be grown from X = Cl at ≥470 °C, Br at ≥500 °C and I at ≥550 °C. Mixed mineralizers of type X = Cl + Br and Cl + I are useful to improve both the yield and the temperature stability range for h-GaN. The size of h-GaN crystals decreases from X = Cl to Br to I. The use of a h-GaN substrate has a phase-stabilizing effect and lowers the temperature stability range for overgrown h-GaN films. The choice of precursor will have an impact on the a and c lattice parameters of self-nucleated h-GaN.