Particle-induced X-ray emission measurements combined with Rutherford backscattering spectrometry (including channeling) have been used to measure directly the total sulphur coverage for the S-passivated GaAs(100) surface. The ion-induced X-ray measurements show that 1.1 ML (1 ML = 6.26 10 1 4 cm - 2 ) of S is found on GaAs(100) after passivation with (NH 4 ) 2 S, while only 0.55 ML of S is present on the H 2 S x -treated GaAs(100) surface. A clearer picture of the S-covered GaAs(100) surface emerges from a consideration of these data in conjunction with current models encompassing surface dimers.