Light and heat stabilizers for synthetic resin compositions and synthetic resin compositions containing such stabilizers are provided, comprising(1) at least one hindered and heterocyclic amine having the formula: ##STR1## wherein: X is a bivalent linking radical forming with the ring structure ##STR2## a nitrogen-containing heterocyclic ring having from five to seven ring atoms, preferably piperidine;R.sub.1 and R.sub.2 are each selected from the group consisting of lower alkyl, preferably methyl, and R.sub.1 and R.sub.2 taken together as pentylene (CH.sub.2).sub.5 ;R.sub.3 and R.sub.4 are each selected from the group consisting of lower alkyl, and R.sub.3 and R.sub.4 taken together as one of butylene--(CH.sub.2).sub.4, pentylene--(CH.sub.2).sub.5, and CH.sub.2 --C(CH.sub.3).sub.2 --NH--C(CH.sub.3).sub.2 --CH.sub.2 --;n is 1 or 2; andwhen n is 1, Y is selected from the group consisting of hydrogen, O.degree., OH, alkyl, alkenyl, alkynyl, aralkyl and acyl having from one to about thirty carbon atoms; andwhen n is 2, Y is selected from the group consisting of alkylene, alkenylene, alkynylene, cycloalkylene and aralkylene having from one to about thirty carbon atoms; together with(2) at least one alkyl thioalkanoic acid amide having the formula: ##STR3## wherein: R is alkyl having from about four to about fifty carbon atoms;R' is alkylene having from one to about three carbon atoms;R.sub.8 is selected from the group consisting of hydrogen, alkyl and aryl having from about one to about eighteen carbon atoms; and when n is 2 and p is 0, bivalent alkylene and oxyalkylene linked to N at two positions;R.sub.9 is selected from the group consisting of alkylene, dialkylene, cycloalkylene, and arylene having from about one to about twelve carbon atoms; and isocyanurate;R.sub.10 is selected from the group consisting of alkylene, alkenylene, thiodialkylene and arylene having from about one to about eighteen carbon atoms;m is 1, 2 or 3;n.sub.1 and n.sub.2 are 0, 1 or 2;p, p.sub.1 and p.sub.2 are 0 or 1;and when p is 1, and n and m are each 1, the sum of m, n and p is three; andq.sub.1, q.sub.2 and q.sub.3 are each 0 or 1:In such combinations, the alkyl thioalkanoic acid amide synergizes the stabilizing effectiveness of the hindered heterocyclic amine.