Differential scanning calorimetry has been used to study the a-relaxation (glass transition) as well as beta-relaxation (sub-T-g relaxation) in both virgin polystyrene (PS) and PS-clay nanocomposite. The beta-relaxations have been measured as a small endothermic peak that appears on heating of the samples previously annealed at temperatures from 30 to 80 degreesC. The relaxations have been characterized by determining the effective activation energies (E) and evaluating the sizes of cooperatively rearranging regions at the glass transition (V-g). The values of V-g have been determined from the heat capacity data. The beta-relaxation in both systems shows comparable values of E that increase with annealing temperature from similar to90 to 170 kJ mol(-1). In the PS-clay system the increase occurs slower, suggesting that this process is more decoupled from the a-relaxation than in virgin PS. The alpha-relaxation demonstrates markedly larger values of E (similar to340 vs similar to270 kJ mol-1) for the PS-clay system than for virgin PS. The PS-clay system has also been found to have a significantly larger value of V-g, 36.7 nm(3) as compared to 20.9 nm(3) for PS. The increase in E and V-g reflects an increase in molecular cooperativity that is introduced by the mutually dependent motion of the polymer chains anchored to the surface of clay sheets.