Significant improvements in radiative efficiency of GaNP epilayers grown on GaP substrates by solid-source molecular beam epitaxy (MBE) are achieved by post-growth rapid thermal annealing (RTA). From temperature-dependent CW and time-resolved photoluminescence (PL) spectroscopies combined with PL excitation measurements, the observed improvements are attributed to annealing out of competing nonradiative centres. This conclusion is supported by the following experimental evidence: reduced thermal quenching of the PL intensity resulting in a substantial (up to 18 times) increase at room temperature (RT) after RTA, and simultaneous improvements in carrier lifetime at RT deduced from time-resolved PL measurements.