An attempt has been made to discover the mechanism of iron cementation in clays by comparing the aggregation effected in dispersed pure mineral clay by its exposure to different stages in the active hydrolysis of a ferric chloride solution. Contact of the clay with the early products of the hydrolysis is apparently necessary for the achievement of stable bonding; clay mixed with the gelatinous precipitate formed at the end of the hydrolysis is not stably aggregated, but a combination of these two phases produces aggregates which are particularly resistant to normal dispersion treatment. The results are discussed in terms of the kinetic steps in the hydrolysis of iron(III) solutions, and their implications for structure formation in soils are outlined.