The system Al2O3-P2O5-H2O contains many phosphate minerals that occur in various geologic environments. The natural occurrence of variscite (AlPO4·2H2O, orthorhombic), including its monoclinic polymorph metavariscite, is largely restricted to soils and aluminous rocks like Al-rich igneous rocks and shales, which interacted with P-rich hydrothermal solutions or groundwater. Variscite dehydrates to berlinite (AlPO4) and a hydrous P-Al-rich fluid. This dehydration reaction AlPO4·2H2O = AlPO4 + 2H2O is, however, metastable, at low concentrations of P in the fluid, because berlinite breaks down at lower temperature to augelite [Al2PO4(OH)3] + H3PO4 + H2O and trolleite [Al4(PO4)3(OH)3] + H3PO4 + H2O. The variscite/metavariscite = berlinite equilibrium has been investigated by synthesis experiments from mixtures of γ-Al2O3 and excess phosphoric acid at pressures between 1 and 5 kbar and temperatures of 100–200 °C using standard cold-seal vessels. The hydration-dehydration equilibrium is mainly controlled by temperature and only weakly by pressure. At 4 and 5 kbar, variscite/metavariscite were found at temperatures of ~150 °C, at lower pressure at 115–125 °C in accordance with their natural mode of occurrence. In this T range, however, variscite/metavariscite is not the sole phase but is always accompanied by variable but generally minor amounts of wavellite and trolleite Al4(PO4)3(OH)3. Secondary wavellite Al3(PO4)2(OH)3·5H2O, hydrated Al-phosphate AlPO4·xH2O (1.1 ≤ x ≤ 1.3) and Al-metaphosphate hexahydrate Al(H2PO4)3 formed during quenching and/or drying of the runs. Berlinite is the reaction product at temperatures 200 °C/4–5 kbar and 150 °C/1–3 kbar and may be associated with augelite and trolleite.