Solvent penetration into nonaqueous dispersions (NAD) of poly(methyl methacrylate) (PMMA) sterically stabilized by PIB were studied by ESR spectroscopy. These colloidal particles were exposed to dilute solution of spin probe 3-carbomoyl 2,2,5,5-tetramethyl 3-pyrolin-1-yloxy and the bimodal distribution of ESR spectra of this probe molecules were carefully monitored as a function of time. Fickian type diffusion of spin probes into the particles was observed and a spherical model was employed to obtain the diffusion coefficients (D) in various solvents. D values were found around 10(-15) cm2/s, and interpreted as the parameter, inversely proportional to the apparent viscosity of the environment inside the colloid particle. Maximum amount of diffused solvent molecules (M infinity) into the PMMA particle was found inversely proportional to polymer-solvent interaction through the solubility parameters. The interconnected network of PIB was found to be responsible for the penetration of spin-probe (i.e., solvent) molecules into the glassy PMMA phase and/or PIB-PMMA interface.