O2SF6 plasma effects on processed GaAs surfaces have been investigated. The influence of plasma parameters such as composition, power and exposure time has been studied. The microwave plasma treatment efficiency has been studied by surface depth profiling (cycles consisting of XPS measurements followed by a slight etching) coupled with an original modelling calculation. We have pointed out that SF6 addition in plasma increases the oxidative rule of oxygen by increasing the total oxide thickness and that the quantity of the different oxidative and passivating species does not limit the oxidative mechanism, in the O-2:SF6 ratio range 80:20 to 40:60. The increase in the plasma power leads to a change in the composition of the outer region of the oxide layer with an increase in the gallium content on the surface, and to an increase in the total oxide layer. The exposure time to the plasma increases also these two phenomena. (C) 2009 Elsevier B. V. All rights reserved.