A composition comprisinga) a chlorinated polymer, andb) at least one compound of formula I ##STR1## wherein X is a group ##STR2## R.sub.1 and R.sub.2 are each independently of the other C.sub.1 -C.sub.20 alkyl, hydroxy- and/or halogen-substituted C.sub.1 -C.sub.10 alkyl, C.sub.3 -C.sub.20 alkenyl, C.sub.5 -C.sub.12 cycloalkyl, C.sub.1 -C.sub.4 alkyl-substituted C.sub.5 -C.sub.8 cycloalkyl, phenyl or phenyl which is substituted by 1 to 3 radicals A.sub.1, which radicals A.sub.1 are each independently of one another selected from the group consisting of C.sub.1 -C.sub.10 alkyl, halogen, hydroxy, methoxy and ethoxy,R.sub.1 and R.sub.2 are further C.sub.7 -C.sub.10 phenylalkyl or C.sub.7 -C.sub.10 phenylalkyl which is substituted in the phenyl moiety by 1 to 3 radicals A.sub.2, which radicals A.sub.2 are each independently of one another selected from the group consisting of C.sub.1 -C.sub.20 alkyl, halogen, hydroxy, methoxy and ethoxy, andR.sub.3 is phenyl or phenyl which is substituted by 1 to 3 radicals A.sub.3, which radicals A.sub.3 are each independently of one another selected from the group consisting of C.sub.1 -C.sub.10 alkyl, halogen, hydroxy, methoxy, ethoxy and (C.sub.1 -C.sub.8 alkyl)oxycarbonyl, with the proviso that at least one of the radicals R.sub.1 and R.sub.2 is phenyl or substituted phenyl as defined above and R.sub.1 is different from methyl if R.sub.2 is phenyl.Some of the compounds of formula I are novel.