Numerous device applications of GaAs are hampered by poor electronic properties of GaAs surfaces and interfaces. Room temperature NH3and H2downstream plasma passivation of native oxide contaminated GaAs surfaces is investigated using attenuated‐total‐reflection (ATR) Fourier‐transform‐infrared spectroscopy (FTIR) and photoluminescence (PL). Using ATR FTIR concentrations of –As–O, –As–H, H–OH, and C–H bonds are monitoredinsituand in real time during exposure of the GaAs surface to H (D) atoms from a microwave discharge through NH3(ND3) and H2(D2). Photoluminescence intensity from the GaAs is monitored simultaneously with the FTIR spectra and used as a measure of surface state reduction. At room temperature, H atoms produced from the discharge remove –As–O and C–H contaminants but not Ga2O3. The appearance of As–H bonds and corresponding increase in PL are delayed significantly from plasma initiation, an increase in –O–H concentration, and removal of –As–O bonds. Because the As antisite defects are concentrated at the GaAs–oxide interface, the rates of As–H formation and PL enhancement may be limited by diffusion of H through the oxide and water layer which forms on the surface. We find that the concentration of physisorbed H2O on the GaAs surface increases throughout passivation. There are two sources of water detected on the surface: (i) reduction of As–O bonds and (ii) reaction of H with the quartz reactor walls. Ga2O3grows on the surface via oxidation of GaAs by the physisorbed water. We surmise that higher H concentration in NH3plasmas results in faster water accumulation on the surface and trapping of the As–H species subsurface. In H2plasma, water accumulation on the surface is slower and trapping of As–H is not observed.