Phase assemblages were determined at water pressures up to 30 kb and temperatures up to 900 degrees C for the following four equilibria: (1) serpentine+brucite=forsterite+vapor; (2) serpentine=forsterite+talc+vapor; (3) forsterite+talc=enstatite+vapor; and (4) talc=enstatite+quartz (or coesite)+vapor. The dP/dT= for equilibria 1, 2, and 4 above 10 kb, and is negative for 3 above 5 kb. The data show that water pressures can be increased to at least 30 kb without the formation of a primary serpentine magma.