Several phosphorus acids (methyl- and phenylphosphonic, diphenylphosphinic, and ethyl phosphoric acids) were found to give esters on reaction with tert-butyldimethylsilanol. The reactions occurred simply on heating the reactants in an inert solvent (hexane or toluene). The reaction is reversible, and yields of esters were improved by continuous water removal. The dibasic acids gave either mono- or disilyl esters; the latter were more easily purified and characterized. The OH groups on the surface of silica gel were also found to react with various phosphorus acids under similar conditions to give surface-bonded esters. Products were characterized by CP/MAS P-31 and Si-29 NMR. In the case of phenylphosphonic acid and ethyl phosphoric acids, the products were spectrally identical with those obtained by phosphorylation of the surface OH groups with the highly reactive 3-coordinate anhydrides of these acids when generated in the presence of silica gel.