A stabilizer for synthetic resins, e.g. polyolefins such as polypropylene, of which the effective ingredient is a mixture of 1,3,5-tris{2-[3-13-t-butyl-4-hydroxy-5-methylphenyl)-propionyloxy]ethyl}isocyanuric acid (I) and at least one sulfur-containing compound (II) selected from compounds represented by the general formulae (II-I) and (11-2):
wherein R 1 is an alkyl group having 4 to 20 carbon atoms, and
wherein R 2 is an alkyl group having 3 to 18 carbon atoms, and R 3 and R 4 are selected independently from hydrogen and alkyl groups having 1 to 6 carbon atoms. Preferably the weight ratio (I):(II) is from 1:0.5 to 1:15.