A fast transient fluorescence technique (FTRF) which uses the Strobe Master System (SMS), is introduced for studying swelling of a cylindrical poly(methyl methacrylate) (PMMA) gel. PMMA gel was prepared by free radical copolymerization of methyl (methacrylate) (MMA) and ethylene glycol dimethacrylate (EGDM). Pyrene (P-y) was introduced as a fluorescence probe during polymerization and lifetimes of P-y were measured during in situ swelling process. Chloroform was used as a swelling agent. An equation is derived for low quenching efficiencies to interpret the behavior of mean lifetimes [tau] during swelling. It was observed that [tau] values decreased as swelling proceeded. The Li-Tanaka equation was used to determine the cooperative, D-c and mutual D-m diffusion coefficients, which were found to be around 10(-5) and 10(-7) cm(2) ns(-1), respectively. (C) 2000 Elsevier Science Ltd. All rights reserved.