In situ resistivity measurements have been performed to monitor the effect of activated oxygen (atomic oxygen, oxygen ions) on films subjected to an electron cyclotron resonance (ECR) oxygen plasma. The resistivity of c‐axis and mixed a+c oriented YBa2Cu3O7-x films, which were deposited in situ by 90° off‐axis magnetron sputtering, was measured as a function of time in an ECR oxygen plasma. The resistivity of a film in the plasma corresponds to that found in the same film subjected to higher oxygen pressure when no plasma is present. The resistivity is shown to be determined by the flux of atomic oxygen. The steady‐state concentration in the film is found to depend on the kinetics of oxygen dissociation at the film surface. From the temperature dependence of the activity of atomic oxygen, we determined the activation energy for the decomposition of O2 at the surface of films with mixed a+c axis orientation and c‐axis orientation to be ∼1.3 eV and ∼2.1 eV, respectively. It is proposed that in the presence of atomic oxygen, the phase diagram of oxygen content versus temperature of YBa2Cu3O7-x is changed in a way that depends on the atomic oxygen flux.