We have studied the influence of the incorporation of As on the optical properties of As-doped GaN layers grown by plasma-assisted molecular beam epitaxy using AS(4) molecules, The arsenic concentration, determined by secondary ion mass spectrometry, is uniform throughout the layers. The incorporation of arsenic depends sub-linearly on the arsenic flux with a log-log slope of about 0.1. The photoluminescence from the As-doped GaN films consists of three main bands, UV excitonic emission at 3.4 eV, UV emission at 3.2 eV and a strong blue band centred at 2.6 eV. The intensity of the blue emission centred at 2.6 eV increases more rapidly with arsenic flux than the concentration of arsenic in the bulk, and has a log-log slope of about 0.49. This suggests that the intensity of the blue emission depends on the concentration of arsenic in the GaN layer approximately to the 4th power. (C) 2002 Elsevier Science B.V. All rights reserved.