We have studied the transition from As-doped GaN showing strong blue emission (similar to 2.6 eV) at room temperature to the formation of GaN1-xAsx alloys for films grown by plasma-assisted molecular beam epitaxy. We have demonstrated that with increasing N-to-Ga ratio there is first an increase in the intensity of blue emission at about 2.6 eV and then a transition to the growth of GaN1-xAsx, alloy films. We present a model based on thermodynamic considerations, which can explain how this might occur. (C) 2002 Elsevier Science B.V. All rights reserved.