Basic oxygen steelmaking (BOS) filter cake has been found to undergo a self-sintering process, improving its mechanical properties to allow easier recycling and utilization on plant. The aim of this study was to gain an understanding of the self-sintering of the BOS filter cake in terms of what reactions occurred, and how strength developed in the filter cake during self-sintering. The approach used was to characterize samples before reaction, and to measure the reactivity of the BOS filter cake during heating in air. Reacted samples were characterized and compared to self-sintered samples from the plant. The BOS filter cake consisted of very fine particles (200–500 nm) of metallic iron and wüstite. Upon heating in air from 100 to 1000 °C, the BOS filter cake underwent a sequence of drying, oxidation, and calcination events. The primary reactions in self-sintering were found to be the oxidation of metallic iron and wüstite to hematite and zinc ferrite, beginning at approximately 120 °C and were largely completed by 500–600 °C. These exothermic oxidation reactions at low temperatures were likely driven by the very fine particle size, and provided the energy required to heat the stockpiles and drive self-sintering. The strength required for recycling the BOS filter cake appeared to result from a network of particle–particle bonds that formed between the very fine iron oxide particles in the matrix during oxidation at elevated temperatures. Temperatures between 600 and 800 °C under oxidizing conditions are likely sufficient to form adequately strong material for transport and recycling in the BOS.