Comparative thermal degradation studies have been made in air and nitrogen on polystyrene (PS), poly(p-methoxystyrene) (PMS), poly(p-hydroxystyrene) (PHS) and p-hydroxystyrene/styrene copolymers (p-HS/S) by thermal analytical methods (TG and DSC). The degradation behavior shown by these polymers in an oxidizing or inert atmosphere is different. PMS and PS, however, degrade in a similar manner in the same test atmosphere, suggesting that similar degradation mechanisms are operative for these systems. Polymers containing p-HS behave differently to PMS and PS in both atmospheres. Their behavior in air is discussed it terms of the antioxidant effect of the phenolic hydroxyl grouping and the related crosslinking reactions. In nitrogen, the anomalous behavior is discussed in terms of the observed carbonization reaction.