Due to its relatively low thermal conductivity characteristics and being a local product, pumice aggregate concrete (PAC) blocks are increasingly used as a masonry wall unit in Turkey. However, there is limited technical and scientific knowledge on hygrothermal performance of PAC blocks. A research project is conducted to study hygrothermal performance of PAC block walls using a calibrated hot box method. Temperature and humidity measurements were carried out at the surfaces of the test wall and in the hollows of blocks on different courses. The results, in general, indicate that the hollows of blocks perform better in terms of thermal transmittance than the lightweight solid parts of them. Noticeable changes are observed in hygrothermal behaviour of the lower and the upper courses depending on the indoor and outdoor climate test conditions. The heterogeneous material and rough surface characteristics of the blocks, in addition to the wet construction type of block wall are the most problematic issues that make measurements and assessments difficult. (C) 2012 Elsevier Ltd. All rights reserved.