Electron spin resonance (ESR) spectroscopy has been employed, together with the cryogenic quenching technique, to monitor occluded radical concentrations during methyl methacrylate-butyl acrylate core-shell polymerizations. Occluded radicals are only detected during the preparation of poly(methyl methacrylate) cores (seeds) presumably because a higher mobility of unterminated chains in poly(butyl acrylate) seeds reduces steady state concentrations. Addition of either butyl acrylate or methyl methacrylate as a second charge of (shell) monomer to a poly(methyl methacrylate) seed results in an immediate decrease in the occluded radical concentration to below that of the detection limit of the electron spin resonance spectrometer. This is due to an increase in chain mobility within polymer particles as a result of their reswelling in the presence of (fresh) unreacted monomer. A return of occluded radical activity is only observed when the second charge of monomer is methyl methacrylate. In systems where occluded radicals are detected, they reach a maximum concentration shortly after the reaction temperature maximum and, thereafter, they decay via a second order mutual termination process.