The application of the third-law method to the determination of the E parameters in cases of free-surface decomposition of MgSO4 and BaSO4 in vacuum and investigation of the effect of oxygen in reactor atmosphere on the rate of MgSO4 decomposition allowed to support the dissociative evaporation mechanism of decomposition of these sulfates with releasing of atomic oxygen as one of the primary products. The experimental values of the E parameter for MgSO4 and BaSO4 are equal to 335.7 +/- 1.7 and 411 +/- 4 kJ mol(-1). The serious difference in decomposition temperatures in vacuum (about 1000 and 1400 K) and the E parameters for these two sulfates is connected with the difference in transfer of condensation energy of oxides to the reactants. In cases of MgSO4 and BaSO4, the values of tau parameter are 0.42 and 0. Dependence of the tau parameter on the ratio of the equivalent pressure to the saturation pressure of corresponding metal oxide has been revealed from a comparison of tau parameters reported in the literature for eight different reactants. This correlation is of prime consideration for understanding of the mechanism of consumption of the condensation energy by the reactant. (C) 2003 Elsevier B.V. All fights reserved.