Measurements of the swelling behaviour of a soil clay, in both Ca- and Na-saturated forms, before and after removal of calcium carbonate have been made. For the Ca-clays, three suctions were used, 10·0 mbar, 63·1 bar, and 1·59 kbar. There were no significant differences in the swelling before and after carbonate removal at 1·59 kbar. At 63·1 bar, removal of carbonate led to decreases in swelling for all samples. At 10 mbar suction, removal led to increases in swelling for samples initially with > 1 per cent carbonate and to decreases for samples with < 1 per cent. For the Na-clays, only 10mbar suction was used and similar results to those for Ca-clays were obtained, except that the swelling was greater and the differences larger. The effect of varying the electrolyte concentration on the swelling behaviour of the Na-clays was also studied. The swelling data can be partly explained in terms of the suppression of diffuse layer formation when calcium carbonate was present. Surface area determinations before and after decalcification led to the conclusion that the carbonate was also acting as a cement. The decrease in swelling after carbonate removal, for all samples at 63·1 bar suction, and for samples initially low in carbonate at 10 mbar suction, is thought to be due to the mobilization and redeposition of hydroxy-alumina polymers during the treatment.