Fourier-transform infrared absorption measurements have been carried out in the two-phonon region of GaAs. Implantation of the nitrogen isotopes 14N and 15N, respectively, into bulk GaAs shows that a local vibrational mode at 471 cm-1 (14N) is due to isolated nitrogen. The band is also found in GaAs1-xNx(0≪x≪0.03) layers grown by solid-source molecular beam epitaxy. The strength of the band correlates quantitatively with the decrease of the lattice parameter determined by x-ray diffraction for x≪0.01 and can be used for the assessment of the nitrogen fraction incorporated substitutionally on anion lattice sites. © 2000 American Institute of Physics.