In situ analysis of hygrothermal state of the Sedlec Ossuary, Czech Republic, is presented in the paper. Within the experimental analysis, the measurement of moisture distribution in several profiles of historical masonry is done on a gravimetric principle. Non-destructive measurement of moisture content on the wall surface is done as well, using a capacitive moisture meter. Moreover, the interior and exterior climate is monitored over an almost two year period. On the basis of the obtained experimental results the hygrothermal conditions in both the masonry and the interior are assessed, including the identification of main moisture sources.