Thermogravimetry and differential scanning calorimetry (DSC) have been used to study the thermal and thermo-oxidative degradation of polystyrene (PS) and PS-clay nanocomposite. An advanced isoconversional method has been applied for kinetic analysis. Introduction of the clay phase increases the activation energy and affects the total heat of degradation that suggests a change in the reaction mechanism. The obtained kinetic data permit comparative assessment of fire resistance of the studied materials. Relaxation kinetics have been measured by DSC and dynamic mechanical analysis. As compared to virgin PS, the clay nanocomposite shows the glass transition at a higher temperature and demonstrates markedly larger activation energy. This suggests that the clay phase lowers the molecular mobility of PS which is another factor contributing to the increased thermal stability of the nanocomposite.