PURPOSE: To obtain the present composition outstanding in thermal stability and chemical resistance, by incorporating a styrene polymer of syndiotactic structure with a phosphorus-based antioxidant, a phenolic antioxidant and a sulfur-based antioxidant in specified amount as a whole.

CONSTITUTION: The objective composition can be obtained by incorporating (A) 100pts.wt. of a styrene polymer mainly having syndiotactic structure with (B) a total of 0.005-5(pref. 0.01-2)pts.wt. of (i) a phosphorus-based antioxidant [pref., tris(2,4-di-t-butylphenyl)phosphite], (ii) a phenolic antioxidant [pref., n- octadecyl-3-(4-hydroxy-3,5-di-t-butylphenyl)propionate], and (iii) a sulfur-based antioxidant [pref., pentaerythritol-tetrakis-(β-lauryl-thiopropionate)].