Synthetic resins, particularly polyolefins, are markedly improved in their resistances to heat and oxidation by the incorporation of a 2-methyl-4-t-butyl-5-hydroxyphenylalkanoic acid ester alone or in combination with a thioether-type antioxidant represented by the general formula ##STR1## wherein R represents an alkyl group of 12 to 20 carbon atoms, or by the general formula ##STR2## wherein R represents an alkyl group of 4 to 20 carbon atoms.