The behavior of volume changes upon solvent swelling and annealing was studied within the interphase domain of poly(methyl methacrylate) (PMMA) particles sterically stabilized with polyisobutylene (PIB). Transient fluorescence technique was applied on these micronsized particles labeled in the PMMA phase with naphthalene (N) groups. Mean decay times [tau], obtained from fluorescence decay measurements, were used to examine volume changes within the PMMA-PIB interphase domain. The mathematical model proposed by Inokuti-Hirayama for the quenching of lifetimes was employed to quantify the individual volume changes and relaxations at this particular domain.