Sorption measurements require an ingenious equipment such as that developed for wood by KO],LMANN and SCHNEIDER [1958]. In KOLL~A~'s device the weight is measured by means of a quartz spring which, due to its outstanding properties, gives very precise values. An equation was developed by this author describing the hygroscopic isotherme in the full range of variation of relative humidity [KoLLMANN 1963]. Quartz springs, however, require a direct reading and thus do not allow a detailed description of the kinetics. It was thought very important, especially with respect to wood plastic combinations, to know the details of sorbate exchange when stating true equilibrium points and to know more about the materials by their exchange rates. Any balance to be used in a sorption equipment requires sensitivity, precision and independence of climate; moreover it must be furnished with transducers for transforming weight into another magnitude to be recorded. We have used strain gauges which are very sensitive to strain and yield a voltage which can be adequately amplified. In the balance strain ganges are glued as Wheatstone bridge on to a thin metal plate (exactly twice as long as the overall length of a single gauge) whose bending is proportional to the weight applied. Bending is amplified by an arm Co achieve an adequate sensitivity. A comprehensive sketch of the balance is given in Fig. 1.