We have studied the nucleation, annealing and growth of GaP on Si substrates. Our findings are very similar to those reported for GaAs/Si. That is, dislocation density after I μm of growth is usually about 108 cm−2 surface morphology is best when a multi-temperature growth process is used and is dependent on the substrate orientation; antiphase domain density is minimized by misorienting the substrates. This commonality of results leads us to conclude that the elimination of interfacial contamination is more important in achieving good epitaxial growth of III-V compounds on Si than is the overcoming of lattice mismatch. In support of this hypothesis we present SIMS data revealing up to 2% of interfacial carbon and TEM observations of an amorphous interfacial phase. The carbon comes from the organometallic source and, we believe, reacts with the Si to form amorphous SiC, which disrupts the coalescence of GaP grains and produces lattice defects.