Hydration equilibria : A(z-)(H2O)(n-1) + H2O = A(z-)(H2O)(n), where z = 1,2, were determined involving ions A(z-) produced by electrospray. An ion source operating at 10 Torr pressure with which thermal conditions can be achieved was used, and the ions were sampled by means of an orifice leading to a mass spectrometer. Free energy values, Delta G(n-1,n)(o), were obtained at 293 K for 16 carboxylic acid anions, RCO(2)(-), and for a total of 17 monoanions of the oxo acids of N, P, S, Cl, and I. The -Delta G(0,1)(o) values for the above lead to an approximate linear relationship with the gas phase acidities, Delta G(ac)(o)(AH), corresponding to the dissociation, AH = A(-) + H+. Therefore, the magnitudes of the Delta G(0,1)(o) values can be interpreted on the basis of the well understood factors which determine gas phase acidities. Determinations of Delta G(n-1,n)(o) for dicarboxylates : HCO2(CH2)(k)CO2- where k = 0 to 8, were also obtained. The hydration exoergicities of these compounds are much lower than those of the RCO(2)(-) anions where R = alkyl. The weaker hydration of the dicarboxylates is due to stabilization of these anions by intramolecular hydrogen bonding which leads to cyclization. Anions where k is low have higher hydration exoergicities, because these anions are less stabilized by the internal H bond which is strained for these species. Hydration free energies for the doubly charged A(2)-anions, CO2(CH2)(k)CO22-, and oxo anions : SO42-, SeO42-, S2O32- (thiosulfate), O3SSO32- (dithionate), O3SC2H4SO32-, O3SOOSO32- (persulfate), O3SS2SO32- (tetrathionate), and 1,5-naphthalene disulfonate were also obtained. The factors affecting the hydration exoergicities of these compounds are discussed.