Ten ancient mortars of dolomitic origin, used in the construction of the church of Santa María de Zamarce in Navarre, Spain, have been studied in order to define their composition and to characterize the type of binder employed. A complete characterization has been carried out including: morphological examination (visually and using optical microscopy); mineralogical studies (X-ray diffraction, XRD); chemical analysis (main components and soluble salts); grain size distribution and thermal studies (thermogravimetric and thremodifferential simultaneous analysis, TGA-DTA). Dolomite and calcite, as binders, and quartz, as aggregate, have been found as themain phases. The important variability of the studied samples has confirmed that the choice of the raw materials and their preparation were not taken carefully. Thermal behavior of the samples has shown the endothermic peaks related to calcite and dolomite decarbonations. No hydromagnesite phases have been detected. Finally, the approximate indications of the original composition of the raw materials mixtures are presented.