Undoped InP epilayers have been grown directly on (100) Si substrates by low-pressure organometallic vapor-phase epitaxy. The surface morphology, x-ray diffraction peak width, and ion backscattering yield each improve substantially with InP thickness (0.1-3 μm). X-ray and photoluminescence (PL) measurements demonstrate that the InP heteroepilayers are under biaxial tensile strain in the surface parallel direction. The carrier concentration profile shows that the carrier distribution in the InP layer is very uniform, while an apparent reduction in concentration occurs at the InP/n-type Si interface. The 77-K PL spectrum reveals a strong near-band-edge emission with a full width at half maximum of 14 meV. Post-growth thermal annealing at 780 °C was confirmed to be effective in improving the overall quality of InP-on-Si. The results presented are superior to those reported previously for InP/Si heteroepitaxy.