mixture of (a) a low-volatility, hydrolytically stable, organically substituted phosphite or diphosphite, wherein the substituent groups are alkyl, aryl or alkylaryl, and said phosphite contains substantially no hydroxy groups, and (b) a low-volatiltiy sterically hindered phenolic antioxidant having alkyl groups at the ortho-positions on the ring with respect to the hydroxyl group and, wherein ...