The mobility of pyrene end-capped polystyrene (Py-PSt) trapped in swollen polystyrene gels (PSt) was investigated. PSt gels were prepared by free radical cross-linking copolymerization (FCC) of styrene (St) and ethylene glycol dimethacrylate (EGDM). The pyrene end-capped polystyrene was produced by atom transfer radical polymerization (ATRP). After drying, these disc-shaped PSt gels were left in a toluene solution of Py-PSt of various molecular weights. During this process, Py-PSt chains were trapped in the gel. These swollen gels were re-dried in air and then immersed in pure toluene solution for monitoring the mobility of Py-PSt chains in and out of the gel. These reswelling experiments were performed at room temperature in real time by monitoring pyrene lifetimes outside (tau(1)) and inside (tau(2)) of the PSt gel by using in-situ fast transient fluorescence (FTRF) measurements. It was observed that tau(2) values decrease as swelling proceeds; however, tau(1) values stay constant during swelling. The Li-Tanaka equation was employed to produce the swelling parameters. The swelling time constant, tau(c) was found to increase as the crosslinker density of the gels and themolecular weight, (M) over bar (n) of Py-PSt chains were increased. It was observed that the collective diffusion coefficient, D, decreased by increasing the molecular weight Mu by obeying D-c approximate to M-1 law. Position of the PSt gel in the fluorescence quartz cell before (a) and after (b) swelling. I-O and I-P are the excitation and the emission intensities.