Mortars of the medieval monastery rain in Pilisszeritkereszt can be subdivided into three definite groups according to their lime/sand ratio. Their use was confined to certain, statically reasonable, points of the building. Hydraulic lime mortars have a high CaO content, yet under-optimum strength, due to the lack of adequate aggregate; their present-day strength is largely a consequene of carbonate bond, but this was achieved after a long time only, presumably after the decay of the building. River sand is inadequate for hydraulic limes, especially in regard to durability. This was well known to the builder but the concession was primarily motivated by the underdeveloped condition of the medieval society. -- AATA