In our previously developed mechanistic framework for dry etching, the three consecutive reaction steps (reactant chemisorption, surface reaction and product desorption) are assumed to be activated thermally and in parallel mechanically by fast particle impacts. According to this model, profiles with perpendicular side walls should be obtained if (a) the rate determining step in the etch mechanism is not the etch product desorption, and if also (b) the rate determining step is activated mechanically only. Both criteria are shown to be fulfilled in the case of oxygen Reactive Ion Etching (RIE) of(photo)resists, which is in accordance with the observed etch profiles. The rate determining step is shown to be most probably the (dissociative) oxygen chemisorption on the resist.