Abstract Sulfonic acid groups attached to macroporous poly(styrene-divinylbenzene) copolymers are considered in terms of accessibility and catalytic activity of model reactions in water and a non-polar solvent. The accessibility is related to a thermodynamic distribution equilibrium or to different local diffusion coefficients. With respect to the morphology in the dry state and the influence of swelling, non-interaction and interaction accessibility can be discerned. Size exclusion, cooperative and hydrophobic effects are relevant to the gel-phase, while pendent polymer chains on macroporous surfaces define the surface accessibility. In apolar solvents this surface region consists of a homogeneous network of acid groups. Their acidity is related to the number of hydrogen bridges forming an association of acid groups, which leads to a high sensitivity towards the amount of sorbed substrate or partial ion-exchange. In protophilic solvents the latter effects do not exist.