Compounds of formula I ##STR1## wherein R.sub.1, R.sub.2, R.sub.3 and R.sub.4 are each 2-ethylhexyl or are independently higher alkyl, or R.sub.1 and R.sub.2 are independently hydrogen or methyl, and R.sub.3 and R.sub.4 are independently higher alkyl are effective processing stabilizers for polymers processed at elevated temperatures by preventing polymer degradation and providing resistance to discoloration. The compounds of formula I also exhibit superior hydrolytic stability and resist moisture pickup on storage.