An operando study of CO oxidation by Ce0.5Zr0.5O2 catalysts under dynamic conditions was performed using a prototype gas flow DRIFTS reaction chamber coupled with a MS. Previous studies conducted under oscillating CO/O-2 feedstream conditions, allowed the identification of carbonates as the C-containing storage species on the catalyst surface, with their decomposition being the origin of CO2 evolution dynamics during O-2 pulses. A deeper comparison of the time-resolved DRIFTS and MS data revealed some differences that were related to the different observation points of the two detectors. Two methods for the determination of penetration depth of IR radiation in catalysts beds were compared and discussed. From the analysis of DRIFTS data, two main pathways for carbonates decomposition were found at the beginning of the bed of the catalysts: one leading to CO2 and Ce(IV) sites and another to CO and Ce(III). The last one is active only above 673 K. The preference of one pathway against the other is strictly related to the Ce(III)/Ce(IV) ratio that shows strong variation during the entire cycle. (c) 2005 Elsevier B.V. All rights reserved.