Molding compositions MC having a reduced tendency to yellow with thermal stress which consist of a polymer PM containing units of formula I: ##STR1## blending from 0.005-1 wt % of at least one reducing inorganic phosphorus compound IP selected from the group consisting of phosphinic and phosphonic acid and its alkali metal, alkaline-earth metal, a aluminum and ammonium salts, wherein the ammonium ion can be substituted with up to four C.sub.1 -C.sub.4 alkyl and/or C.sub.5 -C.sub.8 cycloalkyl groups, with the polymer PM after conclusion of the imidation reaction.