The fast transient fluorescence technique (FTRF), which uses the strobe master system (SMS), was employed to study the swelling of disc-shaped PMMA [poly(methyl methacrylate] gels. Seven gels were prepared by free radical copolymerization (FCC) of methyl methacrylate (MMA) with various ethylene glycol dimethacrylate (EGDM) contents. Pyrene (P-y) was introduced as a fluorescence probe during polymerization. After drying these gels, swelling experiments were performed in chloroform at room temperature. P-y lifetimes were measured from fluorescence decay traces during the in situ swelling experiments. An equation was derived for low quenching efficiencies to interpret the behavior of lifetimes in the gel during swelling. It was observed that P-y lifetimes in the gel decreased as swelling proceeded. The Li-Tanaka equation was used to determine the cooperative diffusion coefficients, D-c, which were found to decrease as the crosslinker content was increased.