Smectites are common constituents of claystones, mudstones and shales and are often present in the caprocks and faults that seal potential CO2 storage reservoirs at depths up to 2 or even 3 km. Whilst the marked swelling behaviour of smectites caused by hydration through interlayer sorption of water molecules has been extensively studied, less is known regarding possible interactions of the mineral structure with supercritical CO2. To explore the possibility of such effects, and to assess any implications for CO2 storage, we performed unconfined volumetric strain (dilatometry) measurements on compacted pellets of montmorillonite, a common smectite frequently present in smectite-bearing shales and fault gouges.