A 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 60 degreesC. Pyrene (P) was introduced as a fluorescence probe during polymerization. Swelling experiments were performed by using P doped PMMA gels under chloroform vapor and in chloroform at room temperature. P lifetimes in and out of the gel were measured from fluorescence decay traces during in-situ swelling experiments. it was observed that P lifetimes in the gel decreased as swelling proceeds. An equation is derived for low quenching efficiencies to interpret the behavior of P lifetimes during swelling. The Li-Tanaka equation was used to determine the cooperative, D-c diffusion coefficients for the gels made at various crosslinker contents. It is observed that D-c values decrease as the crosslinker content is increased both in chloroform vapor and in chloroform.