Summary: The gels prepared by adding ammonium hydroxide rapidly to ferric chloride solutions to various pH values are shown to consist of amorphous material and crystalline goethite; no other iron oxide was definitely identified in the present experiments, although this does not preclude other oxides being present in gels prepared under slightly different conditions. The gocthite grows as acicular crystals on ageing at all pH values, but the rate of growth is greater at higher pH values. The exothermic peaks appearing on differential thermograms of such gels are apparently due to coalescence of extremely minute hematite particles to give larger particles, the sharpness of the peak indicating to some extent the size of the original hematite particles. These observations are related to data for other iron oxides, and the morphology of ferric oxide polymorphs under the electron microscope is considered.