In this work we investigate the evolution of the growth mode and the dependence of the structural and optical properties of III-V dilute nitrides with the growth temperature and the presence of hydrogen. GaAsN epilayers were fabricated on (0 0 1) GaAs substrates by rf nitrogen plasma-assisted chemical beam epitaxy. Various amounts of hydrogen were introduced independently in the growth chamber during the GaAsN epilayer fabrication. Reflection high-energy electron diffraction (RHEED) was used to monitor the evolution of the surface reconstruction during growth, while the interaction of the residual hydrogen with the rf plasma source was monitored, simultaneously and in real time, using optical spectroscopy. The presence of hydrogen and slight variations of the growth temperature resulted in noticeable changes in the surface reconstruction during the initial stages of GaAsN fabrication. For sufficiently high amounts of hydrogen, the RHEED image at the onset of GaAsN growth was characterized by the appearance of three-dimensional features, indicating an increased surface disorder; however, after the subsequent growth of few monolayers, two-dimensional RHEED was recovered for all samples regardless of the amount of residual hydrogen. In addition, samples fabricated under excess hydrogen exhibited an apparent nitrogen content reduction, as indicated by high-resolution X-ray diffraction analysis, and a noteworthy blue shift and signal degradation of their low-temperature photoluminescence spectra compared to samples that were fabricated under lower hydrogen partial pressures. (c) 2007 Elsevier B.V. All rights reserved.