SummaryThe hypothesis that the specific surface of soil can be measured by water sorption is tested with data for 62 subsoils of widely differing origins. Ethylene glycol and water sorption at p/p0=0.47 are found to be very closely related measurements and both are highly correlated with CEC. Both methods give a satisfactory measure of total specific surface for soils classed as smectitic and having a large CEC. However, the application of the multilayer theory to the sorption of water on external surfaces of clayey soils with small CEC suggests that both sorbates overestimate the specific surface of such soils. A better estimate of the errors would be obtained from isotherm measurements with water, which is more suitable for this purpose than ethylene glycol.