A polymer composition comprising a polymer stabilized against the effects of light and heat by the incorporation of a polymer stabilizer, wherein the polymer stabilizer comprises at least one compound selected from the group consisting of compounds of formula (I): ##STR1## wherein R.sup.1 is hydrogen, alkyl, alkoxyalkyl, carboxylic acyl or phenylalkyl, or an optionally substituted tetramethylpiperidyl group; R.sup.2 is hydrogen, alkyl, carboxylic acyl or phenylalkyl group; R.sup.3 is alkylene optionally interrupted by at least one oxygen atom; X is oxygen or --NR.sup.5 --, wherein R.sup.5 is hydrogen or alkyl; Y is --OCO--, --COO--, --CONH-- or --OCONH--; n is 2 to 4; and, when n is 2, Z is alkylene optionally interrupted by at least one oxygen, or sulfur or aryl, cycloalkyl or heterocyclic group, or, when n is 3, Z is alkanetriyl in which a carbon is optionally replaced by nitrogen or, when n is 4, Z is alkanetetrayl optionally interrupted by at least one oxygen atom, and acid addition salts of such compounds.