The conditions of the Pb₂MoO₅ phase formation in PbMoO₄, caused by physicochemical features of PbMoO₄ heating, melting, and crystallization were considered. Preferential evaporation of MoO₃ resulted in excess PbO in the mixture (synthesis) and melt (growth). The study confirms the solid-phase mechanism of the Pb₂MoO₅ phase formation in PbMoO₄ crystals during their cooling below 930°C (eutectic between PbMoO₄ and Pb₂MoO₅).