The formation of substitutional solid solutions between the isostructural gamma-FeOOH (lepidocrocite) and gamma-AlOOH (boehmite) was investigated by X-ray powder diffraction, Mossbauer spectroscopy, electron microscopic and thermal analysis. Samples of gamma-(Fe,Al)OOH were obtained at 15-degrees-C and pH 8 by oxidation of mixed FeCl2-Al(NO3)3 solutions. The unit cell parameters decreased with increasing aluminium content from a = 0.3072 nm, b = 1.253 nm and c = 0.3874 nm, to 0.3053, 1.250 and 0.3858 nm, respectively. The temperature of the differential thermal gravimetry maximum increased with aluminium content up to 10 mol % Al. Quadrupole splitting distribution in the room temperature Mossbauer spectra showed a clear dependence on the concentration of substituted aluminium, enabling determination of the gamma-(Fe1-xAlx)1-y/3O1-y(OH)1+y solid solution limits at 0 less-than-or-equal-to x less-than-or-equal-to 0.1. The cation deficiency in samples responded to y = 0.16 over the whole range of solid solutions formed.