Within the framework of research on ancient mortars in Pavia and its district, thermal. microscopic. and X-ray diffraction measurements have been carried out on some mortar samples taken from three wails situated in the historical center of the city. and belonging to the following periods: 4-6th. 9-10th and the 16th centuries. A careful mineralogical and grain-size analysis showed that local raw materials were regularly used during the Middle Ages. By means of thermal analysis and thermogravimetry it was proved that the Roman age and medieval mortars are richer in calcite, whereas the presence of gypsum and dolomite are indicative of a poorer original composition or of compositional changes induced by atmospheric pollution in the mortars of the 16th century.