The effect of the silica support structure on the characteristics and catalytic properties in liquid-phase hydrogenation of 1-hexene over two types of silica (amorphous SiO2 and MCM-41) supported Pd catalysts was studied under mild conditions. The catalysts were characterized by atomic adsorption (AA), N-2 physisorption, X-ray diffraction (XRD), transmission electron microscopy (TEM), CO pulse chemisorption, and temperature-programmed reduction (TPR). The highest hydrogenation activity was found for the larger pore MCM-41 (d(pore) = 9 nm) supported Pd catalysts. Use as a catalyst support of a smaller pore size MCM-41, a small-pore amorphous SiO2, and a commercial large-pore SiO2 led to lower Pd dispersions. The results suggest that Pd was inside the pores of the large-pore MCM-41 and SiO2, and to a lesser extent for the small-pore MCM-41. However, almost no Pd appears to have been inside the pores of the small-pore SiO2, the support with the greatest percent of pores < 3 nm. There was no evidence of any pore diffusion limitations on reaction on any of the catalysts. TPR results showed that the PdO on smaller pore MCM-41 was not totally reduced at ambient temperature, resulting in a lower amount of active Pd-0 being available for catalyzing the reaction. Leaching of palladium into the reaction media occurred in all cases, and to a significant degree for several of the catalysts. However, the leaching of the Pd seemed to be primarily a function of Pd particle size, with larger particles being more susceptible, and probably particle location. This loss of Pd from the catalyst during reaction was most likely due to formation of palladium hydride, which is known to form more easily on larger Pd particles. Although activity decreased with every reaction cycle, the concentration of surface Pd metal atoms able to chemisorb CO reached a limiting value.