The self-sintering of basic oxygen steelmaking (BOS) filter cake has been studied using in situ high temperature X-ray diffraction (XRD) during heating in air from room temperature to 1 273 K. The aim of the study was to improve the understanding of the self-sintering process, using this in situ method to identify what reactions occur at different temperature ranges. The in situ phase analysis measurements correspond well, and are consistent with, the previously reported characteristics of BOS filter cake oxidation. However, the in situ measurements have allowed a more detailed analysis of the reactions taking place over the temperature ranges studied. Wüstite was the first component of the BOS filter cake to react with air, reacting at temperatures of approximately 373 K to 773 K to form a magnetite-zinc ferrite spinel solid solution. The reaction of metallic iron began at higher temperatures than wüstite, beginning at ~633 K and finishing at ~873 K. The decomposition of fluxes (mainly CaCO3) occurred at temperatures above 873 K. At temperatures higher still, at approximately 1 073 K, hematite reacted with zinc oxide to form zinc ferrite. The knowledge that wüstite reacts at the lowest temperatures is an improvement in the understanding of the self-sintering of BOS filter cake, and may give insights into the initiation of the self-sintering process. This improved understanding will aid analysis and approaches focused on process optimisation to increase the amount of filter cake that can be recycled back to the BOS.