Differential scanning calorimetry (DSC) curves and electrical conductivity α were measured for three forms of zeolite-Y, namely NaY, HY and dealuminated HY (DAY). The DSC curves gave dehydration endotherms for all the forms, whereas the σ values have a number of breaks indicating higher mobilities of the charge carriers. NaY exhibited by far the highest conductivity values, mainly resulting from a large number of Na+ ions in the presence of freely diffusing water molecules. DSC indicated a lower free diffusion of water molecules in the HY form which, together with a relatively weaker acidity, contributes to its lower σ values despite its high water content. The production of a relatively large percentage of Al3+ ions in DAY by dealuminating the mother zeolite HY, together with the presence of freely diffusing water in the zeolitic channels (despite the low water content of DAY), results in its σ values being higher than those of HY.