Fast transient fluorescence technique (FTRF) was employed for studying swelling of disc-shaped poly(methyl methacrylate) (PMMA) gels, which were prepared by free radical copolymerization of methyl methacrylate (MMA) using various ethylene glycol dimethacrylate (EGDM) contents at 60degreesC. Pyrene (P) was introduced as a fluorescence probe during polymerization. Swelling experiments were performed by using P-doped PMMA gels under chloroform vapor. Decay curves of P were measured during in situ swelling experiments. Exponential fits were performed to measure pyrene lifetimes, tau, inside the PMMA gels. It was observed that tau values decreased as swelling proceeded. An equation is derived for low quenching efficiencies to interpret the behaviour of P lifetimes during swelling. The Li-Tanaka equation was used to determine the cooperative diffusion coefficients, D-c, for the gels made at various crosslinker contents. It is observed that D-c values decrease as the crosslinker content is increased. (C) 2002 Society of Chemical Industry.