The results of synthesis experiments in the system KAlSiO4—Mg2SiO4—SiO2—SiO2H2O have been used to outline the melting and sub-solidus phase relations at temperatures from 700 to 1200° C and pressures to 3 kilobars. Studies in this system provide a framework within which petrologic features of the near-surface potassic rocks, some lamprophyres, charnockitic granites, kimberlites, and allied mica peridotites may be discussed. On the basis of the experimental studies the pressure-temperature stability limits of coexisting phases are considered. The bivariant phase relations provide a means by which the olivine ↑ biotite and pyroxene ↑ biotite reaction relations observed in potassic rocks may be accounted for. The phase relations provide a mechanism for crossing the ‘equilibrium thermal divides’ forsterite-potash feldspar and enstatite-potash feldspar, from the silica-undersaturated to the silica-oversaturated region. The petrologic importance of water-undersaturatsed magmas is stressed throughout the discussion.