Accurate measurements of InP refractive indices in the 1300–1600 nm wavelength range are reported. They are obtained using the grating coupling technique within an asymmetrical slab waveguide where an intrinsic InP guiding layer was grown on a doped InP substrate (N=2×1018cm−3). Modal indices are obtained with an absolute precision of a few 10−4from either guided or reflected waves. Bulk refractive indices of intrinsic and N‐doped InP are then deduced from the allowed modes propagating in the layered structure. With sample temperature maintained to within 0.01 K, InP bulk indices are obtained with a typical accuracy of 5×10−4. Furthermore, a (2.02±0.02) 10−4K−1refractive index temperature coefficient has been measured for bulk InP at room temperature. ©1995 American Institute of Physics.