The addition of calcium hydroxide to expansive clays results in beneficial reductions in the swelling characteristics and in increased resistance of the material to deformation under load. In this study, improvements in clay properties corresponded generally to the amount of calcium hydroxide addition. For two clays, each mixed wet with up to 8 per cent of calcium hydroxide and then air dried 14 days, it was found that calcium carbonate was present, that crystalline calcium silicate was absent, and that no calcium hydroxide remained. Leaching tests showed that for additions of calcium hydroxide up to 6 per cent, soluble sodium recovered was less than that recovered from clay to which no addition of calcium hydroxide had been made, and calcium ion utilized was in excess of that expected. In view of the impressive improvement in mechanical properties with increasing amounts of calcium hydroxide additions in excess of the total required for ion exchange, solution, and carbonation, it seems likely that there is another utilization of the calcium ion, perhaps involving the non-clay fraction of the soil. It was expected that exchanged sodium ion would be proportional to the amounts of calcium hydroxide additions. However, there was no evidence of sodium-ion exchange until a suffident excess of calcium hydroxide had been added. It was found that the addition of 8 per cent of calcium hydroxide provided a sufficient excess for one of the two days tested but not for the other, a Wyoming bentonite.