Nitridation processes on GaAs(001) surfaces exposed to plasma-activated N2 were investigated by employing in situ reflectance-difference spectroscopy, reflection high-energy electron diffraction, and in-line Auger electron spectroscopy. We have found that a stable GaN layer is formed only when the As background pressure in the growth chamber is sufficiently low. Nitridation is significantly suppressed under a high background pressure of As. A possible mechanism of this effect and its implication to cubic-GaN growth on GaAs surfaces are discussed. Surface roughening induced by extensive nitridation is also discussed. © 1998 American Institute of Physics.