Effects of high-temperature annealing on the vibrational properties of strained GaAs1-xNx GaAs epilayers with x≤0.037 have been studied by room-temperature backscattering Raman measurements. The reduction in the linear redshift of the LO1 line after the annealing, commonly related to strain relaxation, must be associated with the change in the plasma screening effect in our samples. For x<0.02, the linear tendency of the LO2 line frequency for as-grown samples remains unchanged after the annealing. However, that line frequency deviates from a polynomial behavior to a linear trend after the annealing for x>0.02. This change in behavior has been assigned to the reduction in N-N pairs and concomitantly to the increase in substitutional N promoted by the annealing. © 2008 American Institute of Physics.