GaInNAs grown on GaAs has recently been found to optically emit at wavelengths longer than previously possible with material grown epitaxially on GaAs substrates. To improve radiative efficiency, material is annealed after growth, after which the band gap is found to significantly blueshift. Structural changes that occur during this annealing process were studied using fluorescent X-ray absorption fine structure. By comparing the absorption data with lattice parameter and band structure simulations, it was found that the number of In atoms surrounding N atoms increased after the high-temperature annealing. According to ab initio simulations this ordering will increase the band gap of the GaInNAs alloy. We believe the reduction of free energy drives the reordering process towards increasing In-N coordination. (C) 2002 Elsevier Science B.V. All rights reserved.