Abstract The effect of surface Mo on the water–gas-shift (WGS) activity of Pd/ceria was studied. A series of 1 wt% Pd catalysts, with varying Mo content, were prepared from supports obtained by aqueous impregnation of (NH 4) 2MoO 4 onto ceria. Rates were found to decrease linearly with Mo coverage up to 1.8 Mo/nm 2 and were 10% of that on Pd/ceria after the addition of this amount of Mo. TPD studies with 2-propanol on the Mo-containing ceria demonstrate a relationship between the loss of WGS activity and the ceria sites that decompose the alcohol to propene and water. FTIR measurements suggest that Mo ions exchange with surface hydroxyls on ceria and that carbonates are not formed on ceria surfaces that have 1.8 Mo/nm 2. The results from CO–O 2 pulse measurements suggest that the Mo-containing surface is much harder to reduce than pure ceria. Raman spectra of the Mo-containing ceria show features associated with molybdena only for Mo coverages greater than 1.8 Mo/nm 2. The implications of these results for understanding WGS activity on Pd/ceria are discussed.