In this paper the method of determining moisture distribution in thick brickwork is presented, as it was applied in the case of Royal Castle in Warsaw. In 1974-1977 the author was commissioned to monitor the moisture contents of various walls structures subjected to drying-out treatment. He used neutron absorption method to test effectiveness of the treatment and thermovision to detect areas with excessive moisture, unsuitable for plasterwork. The radiation source used was Pu-Be, rated at 5.45x10*j5*s neutrons/sec, housed in a probe 22 mm d. and 615 mm l. To take measurements a hole 30 mm d. must be drilled at least 400 mm deep. Duration of each measurement is2 approximately.