Moisture changes in egg tempera paintings caused by moisture in their gesso grounds were measured in a closed space by changing relative humidity values between 100 and 32.6%. It was clearly recognized that moisture absorption by paintings depends upon the environmental humidity, e.g. relative humidity values, and the length of time necessary to reach the equilibrium. Hygroscopic changes of components utilized in ground preparation were also investigated. Organic adhesives, e.g. gum arabic, animal glues, or gelatin are very hygroscopic. Cadmium compounds (e.g. CdS or CdSe), HgS, ZnO and chromium oxide green show a little hygroscopicity while umbers, sienna and other earth pigments are slightly hygroscopic and lake pigments and ivory black more hygroscopic.