Abstract The capture and geological CO 2 storage (CCGS) in deep aqui fers, depleted oil and gas fields and coal seams appears to be one of the main solutions to reduce greenhouse gases release to the atmosphere (IPCC, 2005). The geochemical reactivity between the mixed fluids (supercritical CO 2 and brine) and the clayey caprock formations under physicochemical conditions of geological storage remain largely under-investigated. The caprock is characterized by inherent properties notably supported by a high clayey content such as low reactivity, low permeability and high elasticity/plasticity. This study presents experimental and modeling results regarding the geochemical reactivity of two caprock samples under CO 2 storage conditions: a rock sample from the Chinle formation in Moab (Utah–USA) and a sample from the Comblanchie n formation in Charmotte (Paris Basin–France). Experiments were conducted in pressurized cells where fluids (CO 2 and brine) are maintained at constant temperature (80 to 150 ∘ C) and pressure (1 to 150 bar). The duration of the experiments ranged from 30 to 360 days. The brine was synthesized as a representative solution of a saline aquifer. In the experiments, carbonate minerals (dolomite, calcite) dissolve partially or totally in the short term. In the long -term experiments, a new form of complex carbonate precipitates. To evaluate clay minerals reactivity, the same series of experiments were performed with a purified mixed -layer illite/smectite mineral (I/S), illite and smectite. The I/S reveals a clear stability towards CO 2 whereas illite and smectite tend to be destabilized in the long -term. Kaolinite is the most reactive clay mineral as it dissolves totally in the long -term experiments. To predict and compare the geochemical reaction pathways, reactive transport modeling was performed focusing on the be havior of the cap -rock at the timescale of the experiments (30 to 365 days). Calculations were also extended to duration up to 10,000 years which is the relevant timescale for the long term safety assessment of CO 2 storage.