A model of poly(vinyl alcohol) (PVA) hydrogels was developed on the basis of the Ising model. PVA hydrogel pellets were used as supports of a bimetallic catalyst powder (Pd/Sn) which reduces the nitrate ions in drinking water. Hydrogels immobilize the catalyst and reduce the diffusivity of the ions only partly compared to values in pure water. The diffusivities of the relevant ions were calculated by a Monte Carlo approach and measured in a diffusion cell. The calculated and measured values correspond quite well. Diffusion/reaction phenomena within the hydrogel pellets were calculated and measured in a reaction/diffusion cell. The model reproduced the measured satisfactorily.