Specimens of brick and mortar taken from the Hagia Sophia Basilica in Istanbul, Turkey, are being analyzed as part of an investigation of the construction history and present state of structural stability of the building. The analysis, covering both chemical composition and physical properties, involves a suite of instrumental methods including quantitative X-ray diffraction, thermal analysis, and automated image analysis of polished sections under SEM. These conventional methods are being supplemented by neutron diffraction and neutron and Xray small angle scattering. Preliminary results suggest the presence of cementitious materials including plombierite and tetracalcium aluminum carbonate in the mortar as well as calcium carbonate. The mortar contains significant amount of brick dust and also large chunks of brick as coarse aggregate. Given the pozzolanic nature of brick dust, it is possible that the mortar is actually not a conventional carbonated lime material, but rather a proto-concrete. The type of masonry construction involving very wide mortar joints, comparable to the width of the bricks, suggests that the Byzantine architects took advantage of this property of the mortar. This sophisticated materials technology may help to explain how the Basilica was constructed in such a short time period, and also to allow us to understand and model subsequent structural problems, including major deformations of the main piers. Also, the feasibility of dating by radiocarbon and thermoluminescence methods is being investigated to determine which parts of the elaborate buttress system are the original Byzantine structure as opposed to later Ottoman additions.