We have grown GaNxAs1−x layers by molecular beam epitaxy on GaAs(100) substrates using a radio frequency plasma nitrogen source and solid sources for Ga and As. Employing reflection high-energy electron diffraction (RHEED), the GaNAs growth mode was in situ monitored. A three dimensional (3D) growth mode was obtained at the low growth temperature of 420°C. At higher temperatures streaky RHEED patterns were observed during all the GaNAs deposition, indicating a two dimensional (2D) growth mode. The structural and optical properties of the GaNAs layers were studied by employing high-resolution x-ray diffraction, atomic force microscopy, Raman scattering, and spectroscopic ellipsometry. The films grown in a 3D mode presented high density of crystal defects, degraded structural properties, and broad optical transitions. In contrast, GaNAs layers grown in a 2D mode are pseudomorphic with high crystal quality. The properties of samples with a high N concentration were improved by first growing a GaNAs layer with a low N content.