PURPOSE: To obtain a polyetheretherketone compsn. excellent in mechanical strengths and moldability by mixing polyetheretherketone with a specific compd.

CONSTITUTION: 100 pts.wt. polyetheretherketone is mixed with 0.2-15 pts.wt. at least one compd. selected from the group consisting of compds. of formulas I, II, and I (wherein at least one of R1 and R2 is CH3 or C2H5 and the rest is H; R3+ is a monovalent alkali metal ion; and R4" is a divalent alkaline earth metal ion) and molded. Thus, the moldability of polyetheretherketone is improved, and an engineering molding exhibiting excellent mechanical strengths even in slow cooling or annealing is obtd. In the compds. of formulas I, II, and 1, both R1 and R2 are pref. CH3 or C2H5; R3+ is pref. Na+ or K+; and R4++ is pref. Mg++ or Ca++.