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.