A method is described for the determination of the nitrogen concentration in GaP crystals from optical absorption measurements. The method has been calibrated by measuring the absolute nitrogen concentration by nuclear reaction quantitative analysis in the same samples for which the optical absorption was measured. The relationships between the nitrogen concentration, the integrated absorption of theAline (2.3172 eV), and the optical absorption atAx(2.3275 eV) at 4.2°K are given by [N] cm−3=8.5×1014∫&agr;Ad E=3.4 ×1016&agr;Ax, where &agr; is measured in cm−1andd Ein meV. Previous formulas which have commonly been used to determine the nitrogen concentration in terms of optical absorption overestimate the nitrogen concentration by about a factor of 5. Also the application of these formulas has varied between laboratories. Section I describes the determination of nitrogen concentrations by nuclear microanalysis. Measurements have been made in the range from[inverted lazy s]5×1016to[inverted lazy s]1018 cm−3in GaP grown by the liquid encapsulated Czochralski process in a nitrogen‐pressurized crystal puller and in nitrogen‐doped GaP grown by liquid phase epitaxy at [inverted lazy s]1000°C. Two distinct charged‐particle techniques have been employed. In crystals containing nitrogen of normal isotopic composition, the14N concentration was determined by proton activation using the reaction14N(p,&agr;)11C with proton energies in the range 6–10 MeV. The reaction product,11C, was chemically separated after irradiation. The11C is a positron emitter, and the &ggr; rays from the positron annihilation were measured. Boron interference through the reaction11B(p,n)11C was independently determined by measuring the prompt &agr;‐particle emission from the reaction11B(p,&agr;1)8Be*→2&agr;2during irradiation with 685‐keV protons. In15N‐doped LPE layers, the15N concentration was determined by measuring the prompt &agr;‐particle emission from the reaction15N(p,&agr;)12C during irradiation with 685‐keV protons. Section II describes the optical absorption measurements and the correlation with the nuclear measurements. The temperature dependence of the integrated absorption and the line shape of theAline have also been measured. For determination of the nitrogen concentration, theA‐line absorption measurements should be made at a temperature at which the absorption line shape is amenable to accurate integration. The integrated absorption can then be corrected to its value at 4.2°K and the nitrogen concentration calculated from the above relationship.