Silicon oxynitride thin films have been deposited at room temperature on GaAs using the PECVD technique with SiH4, N2O and Ar in a modified magnetron sputtering system. Typical deposition rates were on the order of 350 nm min(-1), substantially higher than has been previously achieved for room-temperature deposition with good optical and mechanical quality. At a fixed ratio of precursor gases SiH4:N2O:Ar = 14:33:160 sccm, bias potential V-B = - 50 V, and total deposition pressure P = 32 mTorr, the atomic ratios of Si/O and N/O were found to be 1.25 and 0.14, respectively, using Rutherford backscattering spectroscopy. From nuclear reaction analysis, the hydrogen content was found to be 6 at.%, much lower than is typical for low-temperature PECVD films. The deviation in the uniformity of the film thickness was within +/- 4% across a 2" GaAs wafer. For the above deposition conditions, the refractive index and the optical band-gap of the films were 1.9 and 2.0 eV, respectively. Raman and photoluminescence spectra show practically no surface damage following film deposition on GaAs/A1GaAs modulation-doped field-effect transistor, unstrained GaAs/A1GaAs quantum-well (QW) structures and pseudomorphic InGaAs/GaAs QW structures.