While it has been recognized that the soil-water retention properties are affected by the soil void ratio, standard test methods for determination of soil-water retention curves do not account for soil volume changes, assuming a condition of constant volume. However, significant volumetric changes are expected when characterizing highly plastic soils, even for comparatively small changes in water content. This paper presents a new testing device that allows continuous measurement of three relevant variables: void ratio, matric suction, and water content. This set up is used along wetting paths in order to determine the soil-water retention surface. The set up allows continuous measurements of the relevant variables by using displacement sensors and a scale system. The new equipment is described in detail, and a set of tests results are presented in order to show its capabilities. The results indicate that, rather than a characteristic soil water retention curve, the new device can be successfully used to characterize a soil water retention surface defined by matric suction, water content, and void ratio [ψ,w,e].