This paper deals with volume changes of an unsaturated soil in the elastic domain. Two constitutive relations are established in the general framework of unsaturated porous media mechanics. They express the variations of the void ratio e and the water content w with respect to the mean net stress (σ − pg*) and the suction s. Associated with validated experimental results, these relations give access to the construction of the state surface e = f((σ − pg*), s) in the elastic domain. This surface shows two domains: the first, near saturation, is based on the hypothesis of a Biot's coefficient bw equal to 1; the second, for high suctions, is based on an experimental value of bw. Next, a preliminary construction of the state surface Gsw = g((σ − pg*), s) is proposed for a given soil. Although restricted to the elastic domain, these constructions give a new understanding of the suction role in important problems related to overconsolidation, shrinkage, or swelling of unsaturated soils.