Interfacial reactions between (100) InP and Pd were investigated as part of a systematic study aimed at investigating the stability of planar nonalloyed metallizations to InP. A 50-nm-thick Pd film was deposited on an InP substrate, and parts of it were subsequently thermally treated for 30 min at temperatures varying from 100 to 500 degrees C in steps of 50 degrees C. Separate characterizations of the Pd, In, and P depth distributions were obtained using mass and energy dispersive recoil spectrometry. The different phases were determined using x-ray diffraction, and scanning electron microscopy was used to study the surface topography. It is assumed that the interaction starts in the as-deposited sample, and definite formation of a ternary phase with the suggested composition Pd5In2P2 starts at an annealing temperature of 100 degrees C. At 250 degrees C all Pd is chemically reacted. Preferential outdiffusion of P leads to a loss of P from the surface at 500 degrees C, and the only phase observed in the x-ray diffraction spectrum from the surface is PdIn.