An apparatus is described with which ion equilibria of the type : M(z+)L(n-1) + L = M(z+)L(n), where M(z+) is an ion produced by electrospray and L is a ligand, can be determined in the gas phase. This opens the door to gas phase ion thermochemistry involving many important ions which can be produced only by electrospray. The ions produced by a conventional electrospray generator at 1 atm are transferred to a reaction chamber which contains a buffer gas, N-2, at 10 Torr and known pressure oft in the milliTorr range, The thermalized ions react and reach equilibrium. The equilibrium ion ratios are determined with a quadrupole mass spectrometer. Equilibrium constants determined with the present technique for M(z+) (Na+, H3O+, Mg2+, n-C3H7NH3+ and L = H2O as well as K+, (CH3N+ and L = CH3COCH3) are found in good agreement with literature values. New determinations for M(z+)(n-alkyl-NH3+, NH2(CH2)(k)NH3+, NH3+(CH2)(k)NH3+ and the protonated peptides Gly(2) H+, Gly(3) H+, Gly(4) H+, Lys H+, Gly-Lys H+, Lys-Tyr-Lys H-2(2+) With L = H2O) are presented and discussed. The magnitudes of the hydration free energies of the protonated peptides can be used to establish whether the unhydrated ions have undergone proton-induced cyclization where the protonated amino group forms a strong hydrogen bond to a basic site on the peptide.