PURPOSE: A composition, prepared by incorporating a specific diphosphonite with a compound selected from an oxalic acid diamide and an organic nickel complex, and having improved heat stability, particularly the suppressed deterioration of molded articles in use for a long term under high temperature conditions.

CONSTITUTION: A composition prepared by incorporating (A) a polyphenylene ether type resin with (B) a compound selected from a diphosphonite expressed by the formula (R1 is 1W10C alkyl; two R1s in one benzene may be the same or different) and (C) a compound selected from oxalic acid diamide, hydroxybenzotriazole, hydroxybenzophenone, a sterically hindered amine and an organic nickel complex. The addition of a sterically hindered phenol is effective. The amounts of the components (B) and (C) based on 100pts.wt. resin are as follows: 0.01W5pts.wt., preferably 0.1W2pts.wt.