Recombination processes in GaP/GaNP core/shell nanowires (NWs) grown on a Si substrate by molecular beam epitaxy are examined using a variety of optical characterization techniques, including cw- and time-resolved photoluminescence and optically detected magnetic resonance (ODMR). Superior optical quality of the structures is demonstrated based on the observation of intense emission from a single NW at room temperature. This emission is shown to originate from radiative transitions within N-related localized states. From ODMR, growth of GaP/GaNP NWs is also found to facilitate formation of complex defects containing a P atom at its core that act as centers of competing non-radiative recombination.