We present a treatment of the cation exchange capacity (CEC) of clays in contact with an aqueous solution, during precipitation, using an ideal solid solution approach. It combines in a single calculation short term processes such as cation exchange and long term reactions like precipitation. This approach has been implemented and optimized in a new version of the NANOKIN code, which is thus now able to provide the time dependence of all particle sizes, composition and precipitated amount, as well as aqueous solution activities, pH and saturation states. It has been applied to the description of montmorillonite and illite precipitation from aqueous solutions at different temperatures and initial conditions: (1) an initial supersaturation with an aqueous solution composition which mimics a fresh water at T = 25 degrees C, a sea water at the same temperature, and an hydrothermal fluid at T = 150 degrees C; (2) a progressive supersaturation resulting from the kinetic dissolution of granitic rock-forming minerals, under weathering or hydrothermal conditions. The competition between illites and montmorillonites known in these various environments is well reproduced, as well as their dominant cation composition and typical particle sizes. (C) 2011 Elsevier Ltd. All rights reserved.