The recent eruption of the Soufriere Hills Volcano in Montserrat (July, 1995, to present; September, 1997) has produced an andesitic dome (SiO2~59--61 wt. %). The eruption has been caused by invasion of mafic magma into a preexisting andesitic magma storage region (P~130 MPa; ≥5 km depth). The composition of the andesite has remained essentially constant throughout the eruption, but heating by the mafic magma increased the andesite temperature from ≤830 ¿C to ≤880 ¿C. Prior to being heated, the stable mineral assemblage in the andesite was plagioclase+amphibole+orthopyroxene+titanomagnetite+ilmenite+quartz. The rise in temperature from ≤830 ¿C to ≤880 ¿C (fO2~1 log unit above NNO) has caused quartz to become unstable, and has also caused changes in silicate and Fe-Ti oxide mineral compositions. The andesitic magma is likely saturated with an H2O-rich vapor phase in the upper part of the magma storage region. Melt H2O content is ~4.7 wt. %. ¿ 1998 American Geophysical Union