Enthalpies of transfer of sodium salts of monomethyl esters of oxalic, malonic, succinic, adipic, fumaric, maleic, o-phthalic, isophthalic, and terephthalic acids from water to water-methanol and water-tert-butyl alcohol mixtures have been determined by calorimetry. Using literature data for the enthalpies of transfer for the sodium ion, based on extra thermodynamic TATE and TPTB assumptions, the enthalpies of transfer of the monoester anions were calculated. The results are discussed in terms of ion-solvent and solvent-solvent interactions.