Degradation of polystyrene into styrene, including monomer and dimer, was studied using various kinds of solid acids and bases as catalysts. It was found that solid bases were more effective catalysts than solid acids for the degradation of polystyrene into styrene. This was attributed to differences in the degradation mechanisms of polystyrene over solid acids and bases. Among the solid bases employed, BaO was found to be the most effective catalyst, and about 90 wt.% of polystyrene was converted into styrene when thermally degraded polystyrene was admitted to BaO powder at 623 K. Polystyrene films with dispersed BaO powder were then prepared as a model of a recyclable plastic using a twin-roller heated at 393 K; the amount of BaO dispersed was only 1 wt.% of the polystyrene films. More than 85 wt.% of the polystyrene films with dispersed BaO were converted into styrene by simple thermal degradation at 623 K without assistance of other catalytic compounds. The films were expanded into polystyrene foams with dispersed BaO using a gas absorption/evolution technique. Styrene could be recovered from these foams by heating them at 673 K.