The objective of this study was to investigate the influence of layer charge on the hydration of Mg-saturated expandable 2:1 phyllosilicates. Water retained by 12 Mg-saturated clays at 54% relative humidity was quantified gravimetrically. X-ray diffraction and total chemical analysis were used to determine the hydratable surface area (447-759 m 2 g (super -1) ) and layer charge [0.327-0.754 electrons per formula unit (e. f.u. (super -1) )] of each sample. Water retained by the clays increased with both hydratable surface area and layer charge of the clays. However, the increase in H 2 O content with layer charge occurred only on external surfaces of the clays. This result suggests that the H 2 O on external surfaces is localized around the cation/charge sites rather than forming multi-layers as was suggested previously. A model is proposed for the hydration of expandable 2:1 phyllosilicates. The model assumes that interlayer volume controls interlayer hydration and that the number of cation/charge sites on external surfaces controls hydration of external surfaces.