Polyphenylene oxide resin composition stabilizer

A polyphenylene oxide resin composition having enhanced resistance to deterioration of mechanical properties during thermal aging, comprising a polypenylene oxide resin and a stabilizing amount of orthosubstituted phenyl acid phosphite represented by the formula ##STR9## in which R.sub.1 is a t-butyl, t-amyl, cyclohexyl or phenyl group;

• R.sub.2 is a hydrogen atom, an alkyl group having 1 to 5 carbon atoms, a cyclohexyl group, or a phenyl group;
• R.sub.3 is a hydrogen atom or a methyl group;
• and R.sub.4 is a hydrogen atom, an alkyl group having 1 to 20 carbon atoms, an ether-alkyl group having 3 to about 70 carbon atoms and 1 to 30 ether groups, an aryl group having 6 to 18 carbon atoms, a cycloalkyl group having 5 to 14 carbon atoms, an aralkyl, alkaryl, or alkoxyaryl group having 7 to 19 carbon atoms, an ethoxylated aryl or alkaryl group having 8 to 78 carbon atoms and 1 to 30 ether groups, or a cycloalkylaryl group having 11 to 19 carbon atoms.