New experimental results are used to constrain the P, T, X(H2O) conditions of the Soufriere Hills magma prior to ascent and eruption. The experiments were performed on a powdered andesite erupted in January, 1996, at an fO2 corresponding to ~NNO+1 with PH2O and temperatures in the range 50 to 200 MPa and 800 to 940 ¿C. Amphibole is stable at PH2O>115 MPa and temperatures 72 wt % SiO2 in residual melt) at PH2O>115 MPa. Analyses of rhyolitic glass inclusions in quartz and plagioclase from recently erupted samples indicate melt water contents of 4.27¿0.54 wt % H2O and CO2 contents <60 ppm. The evolved Soufriere Hills magma would therefore be H2O-saturated at pressures <130 MPa. These results suggest that the Soufriere Hills magma containing the stable assemblage amphibole, quartz, plagioclase, orthopyroxene, magnetite and ilmenite was stored at PH2O of 115--130 MPa, equivalent to a minimum depth for a water-saturated magma chamber of 5--6 km depth. Magma temperatures were initially low (820--840 ¿C). Quartz is believed to have been destabilised by a heating event involving injection of new basaltic magma. The stability field of hornblende provides a useful upper limit (~880 ¿C) for the extent of this reheating. ¿ 1998 American Geophysical Union