We have investigated the optimum annealing temperature of Ga(As,N) layers with nitrogen concentrations ranging from 0.06% to 6.1%. Photo luminescence (PL) measurements show a strong blueshift and an increase in the PL intensity with higher annealing temperatures. The optimum annealing temperature was determined according to the highest PL intensity of the annealed Ga(As,N) samples. We have found that the optimum annealing temperature is nitrogen-dependent and decreases with increasing nitrogen concentration. Raman spectroscopy reveals growth-induced defects in Ga(As,N) that are removed during the thermal treatment. We have also observed degradation of the structural quality during the annealing process by means of X-ray diffraction measurements. From transmission electron microscopy, we were able to directly measure nitrogen diffusion. However, this nitrogen diffusion is not pronounced enough to explain the structural degradation determined by X-ray diffraction. (C) 2004 Elsevier B.V. All rights reserved.