In order to investigate the effects of liquid-liquid phase separation on the igneous evolution of planetary bodies, we investigated immiscibility in the systems CaO-SiO2, MgO-SiO2, and CaMgSi2O6-SiO2 at pressures to 1.5 GPa. Few changes are observed in the size of the miscibility gaps in these systems at 1.0 GPa. The decrease of the stable area of the gaps is mainly due to the increase of the monotectic temperature because the latter closely follows the melting temperature of SiO2 at high pressure. The pressure at which the whole CaO-MgO-SiO2 miscibility gap becomes metastable is estimated to be 1.81 GPa. The thermal and compositional extents of miscibility gaps associated with network-modifier cations (such as Ca2+) do not change significantly with increasing pressure, but the extents are increased for amphoteric cations (such as Mg2+), and the change is more pronounced if the latter have small ionic radii and high ionic potentials. Cations possessing high crystal field stabilization energies produce the largest immiscibility fields at atmospheric pressure, and the passage from the high-spin to the low-spin state with increasing pressure is expected to yield even larger miscibility gaps. Magmas containing substantial amounts of amphoteric cations and cations with high crystal field stabilization energies are therefore potential candidates to develop immiscibility at high pressures.