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Crystallization experiments were carried out on a representative sample of the crystal-rich dacite ejected during the 15 June 1991 eruption of Mt Pinatubo, to define the pre-eruption conditions of this major volcanic event. Experiments were performed in the temperature and pressure range of 750–900°C and 220–390 MPa, respectively. Redox conditions were varied between those of the NNO (nickel–nickel oxide) buffer and 2.7 log fO2 units above (NNO + 2.7). Melt water contents ranged from 3 to 7 wt % (H2O saturation). Phase equilibria at 220 MPa reproduce the phase assemblage of the magma only at temperatures below 780°C and water-rich conditions: melt H2O content >6 wt %, or H2Ofluid > 0.80. Phase abundances and the compositions of hornblende, plagioclase, and melt indicate that the eruption tapped a magma body that was at a temperature of 760 ± 20°C and a pressure in the range 220–180 MPa, with melt H2O contents between 6 and 6.5 wt %, that is, H2Ofluid > 0.88. Thermodynamic calculations constrain CO2 in the melt to be <100 ppm. Comparison between natural and experimental Fe–Ti oxide compositions indicates that fO2 was in the range NNO + 1.5 to NNO + 1.7. Current calibrations of the Fe–Ti oxide thermobarometer are not appropriate for oxidizing conditions, and overestimate both T and fO2 when fO2 is greater than NNO + 1.5. Compositional zoning of plagioclase reflects crystallization over 150°C (900–750°C), and implies conditions close to, or at, H2O saturation throughout crystallization in the upper-crustal magma chamber. Although continuous magma replenishment probably occurred, the injected magma must have had the same bulk composition (including the same melt H2O content) as the erupted dacite to preserve the linear relationship observed between the An and Or contents of the plagioclase. Compositional zoning of hornblende is consistent either with an early crystallization event at ∼400 MPa and 840–900°C, or with mixing events before eruption. The experiments show that addition of S leads to an increase in the mg-number of hornblende in the redox range where pyrrhotite occurs (≤ NNO + 1.4) The lack of Mg-rich overgrowth on hornblende shows that if sulfur was introduced into the magma in the course of its crystallization, such as by volatile infiltration from an underlying, triggering basalt magma, then it must have occurred when the dacite was already oxidized.