The metal-support contact structure of Pd-CeO2 changed with increasing the temperature of reduction. Upon high temperature reduction, severe sintering of Pd particles occurred, while sintering of the ceria support was marginal. The catalytic deactivation of the Pd/CeO2 catalysts during methanol synthesis was caused by the further structural change in the Pd-CeO2 contact under reaction conditions. Considerably large Pd particles (about 100 nm) were formed in the catalysts subjected to methanol synthesis, and there was a close correlation between the activity loss and the growth of the Pd particles. It was proposed that the structure of Pd-ceria contact shifted from small Pd clusters supported on ceria to sintered large Pd particles dispersed in a mass of ceria.