The reaction of dialkyl phosphonates, (RO)2P(O)H where R = CH3, C2H5, and n-C4H9, with hexafluoroacetone gives, in each case, two isomeric products, (RO)2P(O)OCH(CF3)2 and (RO)2P(O)C(OH)-(CF3)2. Products are characterized by elemental analyses, infrared, proton and fluorine nuclear magnetic resonance, and mass spectrometry. The relative yield of isomer depends markedly on the alkyl group present. These results are explained in terms of an inductive effect on the polarity of the P—H bond which influences the direction of addition of hexafluoroacetone.