Oedometer tests were performed on Ypresian clays. The unloading-reloading curves indicate that each path shows bi-linear separated by a threshold stress which is considered as the swelling pressure corresponding to the void ratio just before the unloading or reloading. Upon unloading, when the stress is higher than the swelling pressure, the mechanical effect is dominant with only small mechanical rebound and small microstructure change; when it is lower than the swelling pressure, physico-chemical effect becomes prevailing and soil swells with a larger microstructure change. Upon reloading, when the applied stress is lower than the swelling pressure, the microstructure is not significantly affected thanks to the contribution of the physico-chemical repulsive force, leading to a small volume change; beyond the swelling pressure, the mechanical effect becomes dominant giving rise to larger volume changes corresponding to the microstructure collapse. There is a good relationship between swelling pressure and void ratio just before the unloading or reloading. This is confirmed by the results reported in the literature about other stiff clays such as Boom and London clays. The unloading/reloading loop is rather due to the competition between the mechanical and physico-chemical effects on the microstructure changes than the viscosity as commonly admitted.