2,5-Bisaryl-.DELTA..sup.1-pyrrolines of the formula (I) ##STR00001## in which Ar.sup.1 and Ar.sup.2 are as defined in the description can be prepared by reacting aroylpyrrolidinones of the formula (II) ##STR00002## in which Ar.sup.1 and Ar.sup.2 are as defined in the description with an acid, if appropriate in the presence of a diluent.