We have studied the band-gap reduction of GaAs1-xNx alloys with N contents between 0.1% and 1.5% using the absorption measurements in a series of samples grown by molecular-beam epitaxy. At room temperature, we observed a redshift of the band edge of about 205 meV for 1% of nitrogen. To interpret this effect, we assume that the incorporation of nitrogen in GaAs breaks the symmetry of the system and introduces localized N states that are weakly coupled to the extended states of the semiconductor. We have also studied the temperature dependence of the band gap of GaAsN alloys. The band-gap energy difference between 15 and 300 K decreases from 110 meV for GaAs, to 70 meV for GaAs0.985N0.015. All these experimental results can be explained by the band anticrossing model [W. Shan etal, Phys. Status Solidi B 223, 75 (2001)] with a coupling constant CNM of 2.7 eV, taking into account the contribution of tensile strain to the band-gap reduction. © 2002 American Institute of Physics.