Photoluminescence and absorption spectroscopy experiments are implemented on as-grown and thermally annealed Ga As1-x Nx epilayers grown on GaAs(001) having a nitrogen content in the range of 0.4%-7.1%. At low temperature, photoluminescence spectra exhibit two sets of features: (i) a relatively broad peak at low energy in the vicinity of the band gap predicted by the band anticrossing model (BAC) and (ii) sharp excitonic features at higher energy (over 100 meV above the band gap for x>4%). An enhancement of the photoluminescence response of excitonic emissions and a notable intensity reduction of the deeper luminescence were systematically observed for samples subjected to high-temperature postgrowth annealing treatments. For pseudomorphically strained low nitrogen-containing epilayers (x<2%), and by taking into account the strain magnitude and the average substitutional nitrogen concentration (as extracted from x-ray analysis), excitonic energies and corresponding band gaps (as determined by absorption spectroscopy) are well described within the framework of the BAC model. The extracted binding energies of split heavy- and light-hole excitons are found to be consistent with the expected increase of electron effective masses. For thick partially relaxed epilayers (1%<x<2%) and relaxed epilayers with high nitrogen content (x>4%), the fundamental band gap of GaAsN is found at significantly higher energies than those predicted by the BAC model using the commonly accepted nitrogen coupling parameter CNM =2.7 eV. To account, within the BAC framework, for the apparent deceleration in the band-gap reduction rate requires the use of a smaller coupling constant (CNM =2.0 eV), which suggests a weakening of the strength of the interaction between the localized nitrogen state and the conduction band of the host matrix. This observation seems to be associated with the increasing population of N-related defects. © 2007 American Institute of Physics.