Using different techniques (TG/DTA, XRD and chemical analysis), a comparative investigation on calcitic and dolomitic mortars, from two different Italian buildings of the 11th and 14th centuries, has been performed. By correlating the thermal effects and the steps in the TG curves, it has been possible to reach a semi-quantitative evaluation of the binders in both types of mortars, the results being in good agreement with the chemical data, and to explain the influence of the other components on the thermal behaviour of such complex systems.