This work aims at determining the effect of hydrous strain produced by a continuous, in situ, hydration-dehydration cycles, using a variation of relative humidity (%RH) rate here, variable (%RH), on the cation exchange process in the case of Na rich-montmorillonite. This goal is accomplished in two steps. First, the starting material (Na rich-montmorillonite) is reported "in situ" at variable RH values in order to prepare and characterize a final stressed product that will be used later in the selective exchange study, in the case of solution containing (Cu2+, Co2+). An XRD profile modeling approach is used to describe all structural changes caused by the environmental evolution of the RH rate. The quantitative analysis of XRD patterns is achieved through an indirect method, which is based on the comparison of experimental XRD patterns with calculated ones. This investigation allows us to determine several structural parameters related to the nature, abundance, size, position and organization of exchangeable cation and water molecule in the interlamellar space along the c* axis. (C) 2012 Elsevier B.V. All rights reserved.