Abstract: Determination of the volume of solid bodies by plethysmometry: the bodies, such as human beings or animals, are brought into a measuring chamber (1) the volume of which is changed by means of a piston (8) within a cylinder (7). The change in pressure within the chamber (1) is measured and constitutes a measure of the volume of the body to be determined. The measurement may be performed with the aid of a reference chamber (5) in which the volume is also changed, i.e. by simultaneous movement of a piston (11) within a cylinder (10) together with piston (8) within cylinder (7). By comparison of pressures (by means of 17) between the chambers (1 and 5) a more accurate volume measurement is obtained than without reference chamber (5).