The swelling of disc-shaped heterogels were studied using Fast Transient Fluorescence Technique (FTRF). Disc-shaped heterogels were prepared by combination of methyl-methacrylate (MMA) and styrene (S) with ethylene glycol dimethacrylate (EGDM) as a crosslinker agent in the presence of 2,2'-azobisisobutyronitrile (AIBN). Pyrene (P-y) was introduced as an extrinsic floroprobe during polymerisation. Lifetimes of P-y were measured during in situ swelling processes of heterogels in good solvents to determine the relation between the diffusion and solvent quality. An equation is derived for low-quenching efficiencies to interpret the behavior of lifetime of P-y during swelling. Li-Tanaka equation was employed to determine the cooperative diffusion coefficients, D-c in heterogels, which were found to be strongly correlated with the chosen solvent and the polymeric material in the heterogel system. (C) 2003 Elsevier Science Ltd. All rights reserved.