The elution behaviour of ferrocene, ruthenocene and a series of substituted ferrocenes on a gel permeation chromatography support, cross-linked poly- (acryloyl morpholine) (Enzacryl® Gel), has been investigated. For most of the solutes an approximately linear relationship was obtained between logarithm molecular weight and the Wheaton Baumann absolute distribution coefficient ( K d), indicating that the separations were effected via a molecular sieving mechanism.