In situ methods are ideally suited to study the development of property-critical structural features during material processing. As an example of their potential in the area of complex oxide electroceramics, here, we apply X-ray synchrotron and neutron powder diffraction to investigate the ordering processes responsible for optimizing the microwave dielectric properties of the commercial electroceramic barium zinc tantalate (BZT) doped with strontium gallate. The collection of synchrotron diffraction data with high resolution and high intensity during processing has allowed the development of cation site order and growth of ordered domains to be followed during the multi-stage thermal treatment processing used by industry. Complementary neutron diffraction data reveal changes in the cation-oxygen bonding associated with development of the cation-ordered trigonal perovskite structure. The effects of material loss with processing at temperatures of up to 1525 degrees C are discussed following experiments using open and closed systems. The complex nature of the ordering is confirmed by the coexistence of two BZT phases and the formation of a further zinc-deficient phase. The implications of these observations and relation to the industrial processing procedures are discussed.