The formation and stability of palladium hydride in a well-characterized Pd-on-carbon catalyst have been investigated by using temperature-programmed reduction (TPR), selective carbon monoxide (CO) chemi sorption, and X-ray diffraction (XRD). Under experimental conditions that completely eliminated the possible formation of a Pd carbide phase (PdC) it has been found that the stability of the hydride phase and the H:Pd ratio therein decrease as a function of the percent Pd exposed.