A class of hydrolytically stable bis(aralkylphenyl)pentaerythritol diphosphites is disclosed, which is suitable as an antioxidant additives in polyolefins, particularly, in polypropylene. The diphosphites are of low volatility, have a high thermal decomposition temperature and resist yellowing when blended into a polyolefin base. A preferred diphosphite is bis(2,4-dicumylphenyl)pentaerythritol diphosphite.