Thermoplastic polymers are stabilized against ultra-violet radiation by incorporating therein as a stabilizer a stabilizing amount, e.g., 0.1 to 5% by weight of a tertiary phosphine oxide or sulfide of the general formula: ##EQU1## in which X is oxygen or sulfur and R.sub.1, R.sub.2 and R.sub.3 are each selected from hydroxyalkyl, haloalkyl, carbalkoxyalkyl, acyloxyalkyl, phenylhydroxyalkyl wherein the phenyl radical may be substituted by one or more halogens, and cyanoalkyl wherein the alkyl radical has 1 to 6 carbon atoms.