Summary Surface areas of soil clays can be determined by adsorption of cetyl pyridinium bromide (CPB) from solution. The method is best used with peroxidized, air‐dry samples which have been treated with dithionite‐citrate‐bicarbonate to remove iron and some aluminium oxides, and made homo‐ionic with respect to the exchangeable cation. Apparent CPB areas derived from the plateau of the CPB adsorption isotherm agree with nitrogen areas when no expanding lattice material is present in the sample and when no large proportion of oxides of low surface density of charge remain after the dithionite treatment. When expanding lattice material is present CPB can provide an accurate measure of the internal surfaces provided that the external area is determined independently. Real variations in the crystal size, and hence ratio of external to internal area, are found with montmorillonite, and are exceptionally large for Wyoming bentonite. Specific surface areas determined by the ethylene glycol adsorption method agree reasonably well with areas of non‐expanding lattice clays determined by CPB and nitrogen provided that the specific surface coverage by each adsorbed ethylene glycol molecule is taken as 23Å 2 . When expanding lattice material is present the agreement is less satisfactory, but is improved in most instances by correcting the area on the basis that molecules adsorbed on external surfaces cover 23Å 2 but those on internal surfaces cover 46Å 2 . Within the range of surface densities of charge found for mica‐type materials the adsorption of CPB is much less sensitive to differences than is ethylene glycol, and therefore enables surface areas to be determined more accurately. CPB is also considerably more convenient to use, since well‐defined adsorption plateaux are formed after overnight shaking.