The interfacial reactions between thin films of Co and single crystal InP have been studied. The reaction between the Co and the InP substrate already start in the as-deposited state to form In and P rich compounds. During annealing at 250°C for 30 min., phase separation takes place. In addition to unreacted Co, we find Co rich compounds, a Co-P compound in contact with the substrate, and Co-In compound in contact with the metal layer. The general morphology appears to be stable up to 500°C. At higher temperatures (500–600°C) P is released from the substrate. CoP4, Co2P, CoIn2 and In were detected by X-ray diffraction. At this stage the reaction is no longer uniform.