This paper aims at characterizing hydration behaviour and structural properties of a dioctahedral smectite (i.e. Wyoming montmorillonite), used in the context of naturel barrier for municipal waste disposal sites, and which is saturated by heavy metals cations coming from wastewater solution. Hydration of the <2 mu m size fraction of SWy-2 source clay has been studied by modelling XRD patterns recorded under room and controlled relative humidity (%RH) conditions on Cu2+, Pb2+, Zn2+, Co2+, Cd2+ and Ni2+ saturated specimens. There are heavy metal cations that are characterized by high natural abundance. The qualitative XRD investigation under room conditions (similar to 35%RH) showed a similar hydration behaviour characterized by presence of three principal hydration states: 1W, 2W and 1 W-2 W (i.e. One, two and intermediate water layers hydration state, respectively). In order to highlight the specific hydration properties of each complex, we have studied by quantitative XRD analysis the hydration evolution under controlled relative humidity. Obtained result showed the possibility to distinguish between sample saturated with Na, Cu and Pb for high RH values (approximate to 75%). For Ni, Zn, Cd and Co we have shown similar hydration behavior in all explored RH's range except the Ni2+ cation which presents a homogeneous 3 W hydration state for 75%RH. (C) 2010 Elsevier B.V. All rights reserved.