Palladium (Pd) supported on CeO2-promoted gamma-Al2O3 with various CeO2 (ceria) crystallinities, were used as catalysts in the methane steam reforming reaction. X-ray diffraction (XRD) analysis, FTIR spectroscopy of adsorbed CO, and X-ray photoelectron spectroscopy (XPS) were employed to characterize the samples in terms of Pd and CeO2 structure and dispersion on the gamma-Al2O3 support. These results were correlated with the observed catalytic activity and deactivation process. Arrhenius plots at steady-state conditions are presented as a function of CeO2 structure. Pd is present on the oxidized CeO2-promoted catalysts as Pd-0, Pd+ and Pd2+, at ratios strongly dependent on CeO2 structure. XRD measurements indicated that Pd is well dispersed (particles <2 nm) on crystalline CeO2 and is agglomerated as large clusters (particles in 10-20 nm range) on amorphous CeO2. FTIR spectra of adsorbed CO revealed that after pre-treatment under H-2 or in the presence of amorphous CeO2, partial encapsulation of Pd particles occurs. CeO2 structure influences the CH4 Steam reforming reaction rates. Crystalline CeO2 and dispersed Pd favor high reaction rates (low activation energy). The presence Of CeO2 as a promoter conferred high catalytic activity to the alumina-supported Pd catalysts. The catalytic activity is significantly lower on Pd/gamma-Al2O3 or on amorphous (reduced) CeO2/Al2O3 catalysts. The reaction rates are two orders of magnitude higher on Pd/CeO2/gamma-Al2O3 than on Pd/gamma-Al2O3, which is attributed to a catalytic synergism between Pd and CeO2. The low rates on the reduced Pd/CeO2/Al2O3 catalysts can be correlated with the loss of Pd sites through encapsulation or particle agglomeration, a process found mostly irreversible after catalyst regeneration.