We report the effect of rapid thermal annealing (RTA) on the optical properties of GaN 0.01 As 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 0.01 As 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 (E0+Δ0) are observed. Both of the two transitions increase with increasing annealing temperature.