AbstractHydrodechlorination of 1,2‐dichloroethane on Pd‐Ag alloys was used as a test reaction in order to study mass transfer in carbon xerogel supports. The selected supports were obtained from evaporative drying and pyrolysis of resorcinol‐formaldehyde aqueous gels. Since the pore texture of these materials can be tailored through the adequate choice of the gel synthesis conditions, the maximum pore size of the supports was fixed at 10 × 10−9 m, 30 × 10−9 m (mesoporous supports), or 70 × 10−9 m (macroporous support). One Pd‐Ag/activated carbon catalyst was also prepared for comparison. Diffusional limitations in mesoporous xerogels and in the active charcoal were highlighted by catalytic measurements with pellets of increasing size (≤ 1.125 × 10−3 m); the effectiveness factor of the catalyst decreases when the pellet size increases. On the contrary, it remains equal to 1 for the macroporous support, whatever the pellet size, which shows that diffusional limitations can be completely avoided by choosing an appropriate pore size range. © 2006 American Institute of Chemical Engineers AIChEJ, 2006