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Many magmatic processes, including the extraction of partial melts, fractional crystallization and buoyancy overpressure, depend on the density of the silicate melt phase. Because of its low molecular weight, water has a disproportionately large effect on melt density and accurate data on the effect of water on melt density is required to model density-dependent processes. However, the volumetric properties of the hydrous component at crustal and mantle conditions remain debated. In this study, we determined the density of dry and hydrous (0–9 wt% H 2 O) haplo-andesitic melts from 0.7 to 4.0 GPa and 1600 to 1930 K using the X-ray absorption method in a Paris–Edinburgh press to determine the partial molar volume of water at high pressure. Our in situ data were combined with atmospheric pressure data to derive the first experimental equation of state for andesitic melts valid for crustal and subduction zone …