Subplasticity and aggregate stability have been studied in a group of red-brown earth materials from the Riverine Plain of New South Wales and, for comparison, two samples of krasnozem materials. Within the red-brown earth materials there are positive correlations between subplasticity rating, strongly oriented clay, dithionite-soluble iron oxides, and stability of aggregates; there is no correlation with carbonates. Data on the stability of coarse and fine sand-size and silt-size aggregates under increasingly severe dispersion show that the subplasticity ratings of these materials depend principally on the relative stability of the plasma of the s-matrix, even though bodies of strongly oriented clay are relatively resistant to dispersion. Cementation by iron oxides is discounted as the mechanism causing stability because of the low proportions present in all except one of these red-brown earth materials. The krasnozem materials do not contain strongly oriented clay, so the correlation with subplasticity rating does not hold between groups of different kinds of soil materials. The data suggest that subplasticity in these materials can reasonably be attributed to cementation by iron oxides.