The exchange capacities, surface areas (both external and total), and heats of immersion in water were measured on a selection of clay minerals and a fine-grained muscovite. The charge density (expressed as 103 X m-eq/m2) of muscovite was 3·6. This was, as expected, much higher than those of clay minerals which tended to group together, the average values being: kaolinites, 2·0; hydrous micas, 1·7; halloysites and montmorillonites, 1.4. The heats of immersion (expressed as J/m2) were, in contrast, more widely spread: muscovite, 0·63; halloysites and kaolinites, 0·49; hydrous micas, 0·28; montmorillonites, 0·14. The explanation advanced for the differences is that they reflect changes in surface energy and cohesion of the silicate layers.