The clay size fraction of subtropical soils is dominated by kaolinite, gibbsite, iron oxides, and small amounts of 2:1 clay minerals. Among these minerals. it is more difficult to quantify the presence of the 2:1 clay minerals where kaolinite is the dominant clay mineral present. Thermal analyses of 56 size clay size fractions, free of iron oxides, developed on magmatic rocks, were used to quantify the presence of expandable 2:1 clay minerals. The water content of the samples, measured in three different ranges of temperatures, gave significant (P < 0.0001) linear correlations with the percentage of expandable 2:1 clay minerals and the cation exchange capacities of the whole soil materials. Water content can thus be used as a tool to estimate the quantity of 2:1 minerals in lieu of more complicated procedures. (C) 2003 Elsevier B.V. All rights reserved.