Fast transient fluorescence technique (FTRF) which uses Strobe Master System (SMS) was used to study swelling of disk-shaped poly(methyl methacrylate) (PMMA) gels. Disk-shaped PMMA gels were prepared by free-radical copolymerization of methyl (methacrylate) (MMA) and ethylene glycol dimethacrylate (EGDM). Pyrene (P) was introduced as a fluorescence probe during polymerization, and lifetimes of P were measured during in-situ swelling process. Various organic solvents were used for vapor-induced gel swelling. An equation is derived for low-quenching efficiencies to interpret the behavior of lifetimes, tau during swelling. It was observed that tau values decreased as swelling proceeded. Li-Tanaka equation was used to determine the cooperative diffusion coefficients, D, which were around 10(-5) cm(2) s(-1).