The conductivity of La-montmorillonite was measured in the domain of water relative pressures (p/p0) of <1, and compared with the conductivites of Li- and Na-montmorillonite. La-montmorillonite shows smaller conductivity over the whole range of p/p0 studied. To explain this, theoretical considerations of the polarizing power and of the local stacking order induced by the exchangeable cation were addressed.