Abstract: The application of Time Domain Reflectometry (TDR) technique for measuring moisture content in highly hygroscopic building materials is analyzed in the study. Flue Gas Desulphurization (FGD) gypsum is used as a typical representative of this type of materials. Experimental results show that common methods for calculation of moisture content from measured relative permittivity used in soil science are not applicable for building materials in general. The verification of commonly used conversion functions and mixing formulas by a reference method is necessary case by case.