The interdiffusion between an electroless Ni-P deposit and liquid solder was investigated with the aid of an X-ray diffractometer and a scanning electron microscope to identify the intermetallic compounds formed. Sn was found to be the dominant diffusing atom in the interdiffusion process according to analysis by Auger electron spectroscopy. The intermetallic compounds Ni3Sn4 and Ni3Sn2 were formed between Ni-P and the solder in which phosphorus was also detected. The kinetics of the formation of intermetallic compounds and the activation energy for the diffusion of Sn were investigated.