To study the incorporation of nitrogen, diamond films were prepared by chemical vapor deposition homoepitaxially on {100} and {111} oriented diamond substrates. 50 ppm of isotopic15N2was added to the process gas. Nuclear reaction analysis was applied to determine quantitatively the concentration of incorporated15N. The analysis is based on the detection of the 4.44 MeV &ggr;‐radiation of the15N(p,&agr;1&ggr;)12C reaction. By a proper suppression of the &ggr;‐background, a sensitivity of better than 0.5 ppm can be achieved. The measurements reveal a preferred incorporation of nitrogen into {111} growth sectors, the15N concentration in {111} growth sectors is by a factor of 3–4 larger than in the {100} growth sectors. The N/C ratios in the films were found to be in the ppm regime and four orders of magnitude below the N/C ratios in the gas phase. ©1995 American Institute of Physics.