The results of experiments in which oxygen gas is injected at the current plateau of a quasistationary TFR deuterium plasma are presented. As a consequence of the injection, the molybdenum density in the plasma decreases, apparently because of a decreased molybdenum flux entering the plasma. This density decrease has a time constant which is a measure of the impurity particle confinement time τp. In this way, τp ≅ 50 ms is found at the plasma centre, and τp ≅ 20 ms at the periphery. The discharges studied have been simulated by using different forms of anomalous impurity diffusion. It is concluded that, for the conditions imposed by the experiments, it is necessary to use anomalous diffusion proportional to the impurity density gradients.