We report here the results from recent studies where in situ X-ray techniques have been employed to follow the structural evolution of complex oxide materials from amorphous precursors. Specifically we detail how combined X-ray absorption spectroscopy/X-ray diffraction (XAS/XRD) allowed us to obtain new insight into the heat-induced crystallization of industrially important bismuth molybdate catalysts (Bi2O3 center dot nMoO(3) where n = 3, 2 or 1 corresponding to the alpha, beta and gamma-phases) and nano-sized spinel (CoAl2O4) particles from amorphous zeolites. In addition we illustrate the power of the energy dispersive diffraction technique (EDXRD) to follow the evolution of bismuth molybdate phases in a real autoclave via low temperature (< 150 degrees C) hydrothermal treatment. Finally we illustrate how the results from a combined XAS/XRD and EDXRD study allowed us to propose a chemical reaction leading to the hydrothermal formation of gamma-Bi2MoO6. (c) 2007 Elsevier B.V. All rights reserved.