A procedure is developed for determining the equilibrium constants of reactions of the type:A(solid) double left right arrow B(solid) + C(gas) from TG data. The method is based upon the overlapping of both formation and decomposition TG diagrams when an appropriate heating rate is applied. This procedure has been checked by determining the equilibrium constants at different pressures of the calcium carbonate dissociation products. The method outlined here has been used for determining the thermodynamic parameters of the following reaction:Pb3O2CO3 double left right arrow 3PbO + CO2The results obtained suggest that the phase diagram reported in literature for the PbO-O-CO2 system should be reviewed.