Hagia Sophia has been suffering from salt crystallization and flaking of the wall surface,especially at the northwest part of the building. Therefore,a survey was conducted with the following aims: 1. To identify the crystalized salts in Hagia Sophia 2 .To clarify the origin of salt and the movement of water which is the main cause of salt crystallization 3. To make a suggestion for the future conservation program,including salt and water control. Elements containing salt samples were detected by X-ray fluorescence spectroscopy, and crystallized molecules were identified by X-ray diffraction analysis. As a result,the types of crystallized salt were identified as sodium sulfate,magnesium sulfate and sodium nitrate. At the northwest part of the building (northwest exedra), the major type of salt is sodium sulfate;magnesium sulfate and sodium nitrate were found atthe western and northern parts of the exedra where salt crystallization is in progress. The removal of plaster from the outer wall surface and the infiltration of rainwater from the exposed wall are thought to be the causes of deterioration at the northwest part of the building
