The redox behavior of Pd/CeO2-ZrO2 (Pd/CZ) model three-way catalysts with different particle sizes was studied by in situ diffuse reflectance spectroscopy under the lean-rich perturbation condition. To estimate the contribution of three-phase boundary (TPB) between Pd, CZ, and gas phase, a kinetic analysis of the Pd redox was performed using time-resolved reflectance data. Because the Pd oxidation upon rich-to-lean switching was strongly retarded by oxygen storage in CZ, the ratio of apparent initial rate constants of the Pd redox, k(R -> L)/k(L -> R), was significantly correlated with the presence of TPB. When both Pd and CZ particles are so small that most Pd is close to TPB, the retardation effect is most obvious (k(R -> L)/k(L -> R) similar to 0.3). Increasing particle sizes of not only Pd but also CZ reduces the abundance of TPB and thus the retardation effect is less pronounced (k(R -> L)/k(L -> R) > 1). The rate constant ratio provides a useful index of the degradation of TPB to estimate the extent of deactivation of the three-way catalyst via thermal sintering.