Mechanisms of chain termination have been investigated in two typical heterogeneous systems: on the one hand in the polymerization of monomers adsorbed by an aerosil surface, and on the other hand adsorbed in the polymer matrices. This particular study centres on the phenomenon of “immobilization” of active centres in methyl methacrylate polymerization processes. A number of physical phenomena having a possible role to play in the studied effects are discussed. Experimental data on some properties of immobilized PMMA radicals (including molecular mobility, reactivity and others) are also discussed.