Small molecule (1-naphthylmethyl pivalate) (NMP) diffusion into poly(methyl methacrylate) (PMMA) particles sterically stabilized by polyisobutylene (PIB) were studied by the transient fluorescence technique. Mean lifetimes of NMP increased as it diffused into the interphase region indicating that diffusion is accompanied by quenching of excited NMP molecules by PIB phase. A delayed Fickian diffusion model is fitted to experimental data and a diffusion coefficient is measured around 10(-19) cm2/s at room temperature. PIB density distribution in the interphase domain is found to be Gaussian. A supporting experiment is performed in Anthracene labelled PMMA particles using the direct-energy transfer method. Reversed Gaussian distribution is detected for the PMMA density in the interphase domain.