A process for the manufacture of acicular ferrimagnetic iron oxide by heating iron(III) oxide or iron(III) oxide hydroxide at 220.degree.-460.degree. C. under a water vapor partial pressure of not less than 30 mbar, reducing the resulting product to magnetite by means of hydrogen and/or CO and/or an organic compound which can decompose in the presence of iron oxide, and then oxidizing the magnetite with an oxygen-containing gas to give acicular ferrimagnetic iron oxide of the formula FeO.sub.X, where X is from 1.33 to 1.50.