As part of an ongoing research project to better understand the hygrothermal performance of solid masonry brick buildings, the material properties of dozens of historic fired clay bricks have been measured. This paper discusses the relationship between density, porosity, capillary and saturation moisture content as well as thermal conductivity. A clear relationship was found between bulk density and porosity, indicating a similar mineralogic basis for all fired clay bricks. Also capillary and saturation moisture content showed a clear dependency on the density. For the other material properties the dependency was less outspoken.