The formation mechanisms of the complex Ca-rich ferrite iron ore sinter bonding phases SFCA and SFCA-I, during heating of a synthetic sinter mixture in the range 298–1623 K and at pO₂ = 0.21, 5 × 10^–3 and 1 × 10^–4 atm, were determined using in situ X-ray diffraction. SFCA and, in particular, SFCA-I are desirable bonding phases in iron ore sinter, and improved understanding of the effect of parameters such as pO₂ on their formation may lead to improved ability to maximise their formation in industrial sintering processes. SFCA-I and SFCA were both observed to form at pO₂ = 0.21 and 5 × 10^–3 atm, with the formation of SFCA-I preceding SFCA formation in each case, but via distinctly different mechanisms at each pO₂. No SFCA-I was observed at pO₂ = 1 × 10^–4 atm; instead, a Ca-rich phase designated CF_AlSi, formed at 1420 K. By 1456 K, CF_AlSi had decomposed to form melt and a small amount of SFCA. Such a low pO₂ during heating of industrial sinter mixtures is, therefore, undesirable, since it would not result in the formation of an abundance of SFCA and SFCA-I bonding phases. In addition, CFA phase, which was determined by Webster et al. (Metall. Mater. Trans. B, 43(2012), 1344) to be a key precursor phase in the formation of SFCA at pO₂ = 5 × 10^–3 atm, was also observed to form at pO₂ = 0.21 and 1 × 10^–4 atm, with the amount decreasing with increasing pO₂.