This paper reports on the use of high-temperature X-ray diffractometry, in conjunction with differential thermal analysis, to study the solid-state reactions and phase transformations involved in and leading to the synthesis of various (bismuth oxide-based) Sillenite-type compounds. These, which have gamma-body centered cubic (bcc) structures related to gamma-Bi2O3, are generally represented by the formula Bi12MO20; here, the second cation M = Ge, Si, Ti, Zn, Al, as well as the two-cation combination {Si0.5Ge0.5} . In each case, using the two techniques, measurements were performed on starting powder-mixtures of bismuth and cation M oxides, formulated with desired proportions to favor the Sillenite (gamma-)phase formation. Henceforth, the proceeding of solid-phase reactions and occurring of phase transitions, with rising temperature, were monitored and elucidated by the evolution of the X-ray diffractometry patterns and the appearance of thermal events in the differential thermal analysis curve, as both sets of results were also correlated. (C) 2003 Elsevier Science Ltd. All rights reserved.