High-resolution Brillouin spectroscopy experiments were carried out to determine the velocity of surface acoustic waves (SAW) as a function of composition in GaAs1-xNx/GaAs(100) epilayers (0 < x < 0.037) grown by organometallic vapor phase epitaxy. Experimental data were analyzed using simulations of surface acoustic wave velocities. For all compositions investigated, SAW velocity measurements in both the [100] and [110] crystallographic directions are in close agreement with values predicted assuming linearly interpolated elastic constants between those of GaAs and GaN. (c) 2007 American Institute of Physics.