The mortars and 'cocciopesto' plasters were largely used in the historic buildings. as waterproof coverings and in the preparation of mortars, especially in horizontal structures. In recent ears. the necessity of restoration interventions with materials which are compatible with the masonry structure requires a detailed study on these materials. In this study. the composition of a significative number of mortars of the Byzantine period in the Hagia Sophia Basilica are investigated. They Here made of 'cocciopesto' (crushed ceramic) and probably have a bearing function in the masonry structure. The samples are examined by particle size. infrared spectroscopy (FT-IR) and thermogravimetric analyses (TG-DTG), both on the total sample and on the finer fraction (<63 mu m) which mostly constituted of binder. The results indicate slightly hydraulic mixtures made with a binder/aggregate ratio variable in volume between 1/2 and 1/4. The mixture morphology and heterogeneity seem to imply the use of hot lime technology that would lend themselves es to the bearing function of the mortar