The preparation of chain extended cyclohexanone-formaldehyde and acetophenone-formaldehyde resins and their physical properties were studied. The chain extension was regulated by the ratio of the hydroxyl groups of the ketonic resin/reactive reagents. Both resins were chain extended with dimethyl dichlorosilane, phosphorus oxychloride, phenylphosphonic dichloride, toluene-2,4-diisocyanate, prepolymers (prepared from trimethylolpropane and toluene-2,4-diisacyanate), phthalic anhydride, tetrahydrophthalic anhydride, trimellitic anhydride, 4,4＇-oxydiphthalic anhydride, and maleic anhydride. Solubilities, melting point, molecular weight, and flammability of the chain extended resins were affected by the extender reagent.