Addition of aluminium to the ferrous sulphate mother solution used to synthesize lepidocrocite, γ FeOOH, yielded a substituted, though excessively hydrous (Fe,Al)OOH structure. Apart from reducing X-crystallinity, the main effect of isomorphous incorporation of aluminium in the lepidocrocite lattice was a shift, to higher values, in the temperature at which the lepidocrocite dehydration to maghemite was complete. Increased thermal stability of this substituted lepidocrocite, deduced from measurements of magnetic attraction made after serial heating, was consistent with the presence of a boehmitic component in solid solution. Magnetic susceptibility data for an oxisol indicated that a similar ferri-aluminous complex had developed pedogenically in the upper solum. This constituent was, however, amorphous to X-rays.