High-resolution X-ray diffraction (HRXRD) and secondary-ion mass spectroscopy (SIMS) were used to measure the N compositions of a series of as-grown GaNAs samples grown by solid-source molecular-beam epitaxy. We found that for the coherent samples, N compositions measured by the two methods agree well at lower N compositions (x < 3%), deviate at larger N compositions (x > 3%). The HRXRD measurement by using Vegard's law to extract the lattice constant of GaNAs underestimates N composition at larger N compositions. We found that the underestimation is 8.1% at the x(SIMS) = 3.7%. Partially strain relaxed samples were also studied by using (1 15) XRD mapping. The strain relaxation may cause N composition underestimated by XRD. (C) 2004 Elsevier B.V. All rights reserved.