Photoluminescence (PL) imaging and spectroscopy have been used to investigate isolated, individual indium phosphide nanowires (InP NWs) at both room temperature and 7 K. PL images and spectra show that the emission maxima, line shapes, and intensities are nearly identical along the axis of a given NW. PL spectra collected on InP NWs with diameters of 10. 15. 20, and 50 nm show that emission maxima systematically shift to higher energy with decreasing wire diameter, for diameters less than 20 nm. An effective mass model (EMM) has been developed using particle-in-a-cylinder wave functions for electrons and holes to explore quantitatively the diameter-dependent PL data. The EMM provides excellent fits to the diameter-dependent data obtained at both room temperature and 7 K, and shows that the shifts in the emission spectra can be explained by radial quantum confinement.