Functional gel-type resins (OFP) composed of 2-hydroxyethyl methacrylate (HEMA, 20 mol%), styrene (60-77 mol%) and diethylene glycol dimethacrylate, the crosslinking reagent (DEGDMA, 3-20 mol%) are used as supports for palladium catalysts (0.25-2 wt% Pd). The influence of polymer mass expansion on the activity of Pd/OFP catalysts in the hydrogenation of cis- and trans-isomers of unsaturated dicarboxylic acids, maleic (MA) and fumaric (FA), is examined. Starting polymer swells very well in THF and this solvent is used in the hydrogenation experiments. Swelling ability of polymer decreases after insertion of palladium ions due to the crosslinking effect. Reduction of Pd/OFP by NaBH4 leads to further decrease in swelling ability of polymer caused by the changes in polymer structure observed in FTIR spectroscopy. In the reduced form of the catalysts the nano-particles of Pd (2-3 nm) are formed. Hydrogenation experiments performed in a wide range of operating conditions (Pd loading, catalysts concentration, crosslinking degree of polymer, the size of catalysts grains) show that the swelling of polymer mass during the hydrogenation is a crucial factor for activity of Pd/OFP catalysts. When the catalyst does not swell, the rates of hydrogenation for both isomers are practically the same. They differ only when the catalysts grains are in swollen state. Much higher rate of hydrogenation is observed for MA (the cis-isomer) than for FA (the trans-isomer). This is related to easier penetration of cis-isomer (MA) inside the bulk of the polymer mass which results in more effective utilization of substrates, MA and FA, when present in THF both enhance the expansion of polymer mass but especially cis-isomer, MA. Substrate effect is attributed to the interaction between functional groups present in the reduced catalysts and the hydrogenated reagents, maleic or fumaric acids. (C) 2008 Elsevier Ltd. All rights reserved.