Degradation and relaxation studies have been performed on two polystyrene (PS)-montmorillonite clay nanocomposites, one of which has an intercalated PS-clay structure and the other an exfoliated PS-clay brush structure. Compared to virgin PS, both nanostructured materials have demonstrated the following similarities: (a) a high yield of alpha-methylstyrene in the degradation products as measured by infrared spectroscopy; (b) larger values of the activation energy of the thermal degradation as determined by isoconversional kinetic analysis of thermogravimetric data; and (c) larger values of the activation energy for the glass transition as found from the frequency dependence of the glass transition temperature measured by multifrequency temperature-modulated differential scanning calorimetry. These effects are taken as structure independent manifestations of nanoconfinement of the PS chains in the PS-clay materials. Heat capacity measurements have been employed to evaluate the size of the cooperatively rearranging region, VCRR, that is found to be structure dependent. Compared to its value in virgin PS, VCRR has markedly increased in the exfoliated PS-clay brush system but remained practically unchanged in the intercalated PS-clay system.