Summary: Synthetic mixtures of CaO and Fe2O3 fired in air at temperatures in the range 1180° C to 1240° C in some cases lost oxygen and formed ferrous-iron-bearing compounds. Samples after firing were examined by incident light microscopy and X-ray diffraction; new powder data are presented for the phases reported. In addition to α-Fe2O3, CaFe2O4, and Ca2Fe2O53. four distinct phases have been identified. These are CaFe4‴O7,Ca7·2Fe0·8″Fe30‴O53, and two forms of (Ca,Fe″,Mg)4Fe14‴O25, described previously by Braun and Kwestroo (1960). The likely compositions of these phases are given.