Experimental results of isotropic stress tests performed on statically compacted clayey silt are presented to examine the volume change response of the material subjected to controlled increments in net mean stress and suction within the elastic domain. The results were interpreted with the help of a single effective stress equation –expressed as the sum of two components: the net stress and a fraction of suction– and enabled the determination of the constitutive parameter X entering into the fraction of the prevailing suction. Volume change results on this medium plasticity soil suggest that the proportion of suction contribution to the effective stress is smaller than predicted by the term ‘suction times degree of saturation’ (i.e., X = Sr), generally used in the effective stress expression of Bishop type. Experimental results were fitted to a power law, X = Sr N, where N is a parameter (N > 1) that changed the effect of the degree of saturation. This power law takes microstructural effects into consideration.