Measurements were made of the dynamics of CO oxidation over a series of fresh and aged CeO2-ZrO2 catalysts of different composition. In order to evaluate the varying contributions to oxygen storage/release capacity, we carried out the reaction under transient "anaerobic environment" tin both pulse and cycling mode) and steady-state conditions. The presence of ZrO2 was of little or no benefit to stoichiometric CO oxidation carried out under stationary conditions. Under these conditions, catalytic activity is strongly dependent on surface area and on the number of exposed cerium atoms. The highest performance level is achieved with pure ceria. The beneficial effect of CeO2-ZrO2 mixed oxides on catalytic performance was observed when CO oxidation was carried out in transient pulse mode or under cycling feedstream composition. In this case, catalysts having the composition CexZr1-xO2 with 0.5 < x < 0.8 show an increased catalytic effectiveness correlated to their oxygen storage capacity and redox activity. The promotional effect of ZrO2 was much more evident after severe aging. The decrease in oxygen storage activity with pure CeO2 was higher than that observed for ceria-zirconia and was mainly correlated to the drop in surface area. In contrast with ceria, the low-temperature activity of CeO2-ZrO2 solid solutions for CO oxidation under anaerobic conditions was positively dependent on the degree of reduction of the material. A quantitative estimate of the contribution of bulk diffusion to the overall performance was carried out by calculating the O-2-diffusion coefficient. These values were calculated from conductivity measurements using the Nernst-Einstein relation. It is shown that, in the temperature range investigated, bulk diffusion was approximately two orders of magnitude higher for ceria-zirconia than for ceria and may have been responsible for the enhancement of CO conversion observed with respect to ceria-zirconia-based catalysts.