The synthesis of multiferroic BiFeO 3 has been undertaken in situ using both vacuum and argon atmospheres. We have collected and refined synchrotron X-ray powder diffraction data from pressed pellets of the starting powders of Bi 2 O 3 and Fe 2 O 3 at intervals in the reaction to form the final multiferroic BiFeO 3 product. Data were collected at 3-min intervals following as realistically as possible the sintering regimes used in industry. The difference in the transformation temperature of monoclinic Bi 2 O 3 to cubic Bi 2 O 3 was found to be 650 and 700 β’ C when samples were sintered in a vacuum and argon environments, respectively. It was found that this reaction was 75% complete before the multiferroic product began to form. In both cases it was found that the quantity of Fe 2 O 3 was unaffected by increasing temperature until after the transition of monoclinic to cubic Bi 2 O 3 had reached its maximum value. After this transition the quantities of cubic Bi 2 O 3 and Fe 2 O 3 were found to decrease at very similar rates yielding the final BiFeO 3 structure. An SEM study of the bulk microstructure of the BiFeO 3 product showed a poor densification due to incomplete reactions between the remaining Bi 2 O 3 and Fe 2 O 3 .