In this paper, a non-calorimetric method for measurement of the enthalpies of complex formation reactions from temperature coefficients of the metal ion activities (or equilibrium concentrations) has been developed. The reactions of complex formation of an arbitrary composition m n M L in aqueous solutions can be represented by the following scheme:  schemeNear the reaction equation the related thermodynamic characteristics are written, namely, the equilibrium constant T mn  and the enthalpy of the complex formation reaction T mn H at the temperature T (below this index is omitted). Within the method the mn H calculation is performed according to the equation of reaction-isobar or Gibbs-Helmholtz equation:schemeTherefore, for the mn H determination it is necessary to know the dependency on temperature of the equilibrium constant or Gibbs energy change. The equations for calculating the dependence of temperature coefficients of the degree of complex formation reactions α on the variation of enthalpies of complex formation reactions have been derived. The processing of experimental data can be practically carried out by numerical methods, if for the initial composition of reactant mixture there are not less than five experimental values ln (or  ln[M] ) for different temperatures. To determine mn H at temperature T it is necessary to know the set of equilibrium constants at this temperature and the dln /dT values, which are calculated using numerical methods from experimental data ln α = f(T) or ln[M] = f(T), where [M] is the equilibrium concentration of metal ion. Deduced equations allow the approximate estimation of the actual error of the mn H determination. Thus, the error in the mn H  0.5 kJ/mol in the range of 300 K and the temperature variation interval of 10 degree corresponds to the ln[M] or ln value variation of 0.007 log units. Computation of enthalpies of the series of complex formation reactions from temperature coefficients of the degree of complexation has been performed. Comparison of the measured exp mn H values by the calorimetric method with the calc mn H values obtained by the developed method gives a quite satisfactory agreement.