A study of the interfacial properties of wafer bonded InP/Si is presented. The electrical properties are measured by bonding InP to Si with different doping concentrations and by measuring the current voltage characteristics of the bonded pairs. Two different kinds of structures are studied: 1) n+InP doped to 3×1018 cm-3 bonded with n+Si doped to 1×1019 cm-3, and 2) n+InP doped to 3×1018 cm-3 bonded with p-Si doped to 6×1014 cm-3. After a 350°C anneal, structure 1 shows low resistive ohmic behavior while structure 2 is rectifying due to its higher sensitivity to interfacial chemistry. Indeed, both the reverse and forward current are reduced by annealing in structure 2, suggesting the formation of an interfacial barrier during annealing. This hypothesis is supported by High Resolution Transmission Electron Microscopy combined with Fourier Transform Infrared Spectroscopy, which indicate the formation of an amorphous interfacial oxide from the reaction of adsorbed water trapped at the InP/Si interface.