Abstract We report the effect of rapid thermal annealing (RTA) on the optical properties of GaN 0.01As 0.99 samples grown by molecular beam epitaxy. In particular, a blueshift of the PL peak energy is observed when annealing the samples at 650–900 °C. Samples annealed showed pronounced enhancement in PL intensity as compared to the as-grown sample, and 850 °C is proposed as the optimum RTA temperature. The results are examined as a consequence of RTA-induced nitrogen diffusion inside the GaN 0.01As 0.99 material rather than diffusion out of the alloy, which homogenizes initial nitrogen composition fluctuations. In addition, for photo-modulated reflectance (PR) spectra of RTA samples, the fundamental band gap transition ( E 0) and the transition from the spin–orbit split-off valence band ( E 0 + Δ 0 ) are observed. Both of the two transitions increase with increasing annealing temperature.