Bi2WO6 has attracted considerable interest as a visible-lightdriven photocatalyst with a layered Aurivillius structure. The catalytic performance of bismuth tungstate is notably enhanced through the partial substitution of tungsten with molybdenum. Whereas hydrothermally obtained Bi2W1-xMoxO6 solid solutions maintain the Bi2WO6 structure, their morphologies vary with the molybdenum content. Lower Mo contents (x < 0.5) favor the formation of hierarchically nanostructured microspheres that consist of sheet-like building blocks. Their disintegration is observed for greater extents of W/Mo substitution, hand in hand with a decrease in the surface area. Raman spectra furthermore indicate changes in the local structure of the octahedral W/Mo moieties upon variation of the Mo content. As little is known about the growth kinetics and hydrothermal formation processes of nanostructured Bi2W1-xMoxO6 catalysts, in situ XAS investigations were performed to determine the onset of their hydrothermal formation from Bi(NO3)(3)center dot 5H(2)O, K2WO4 and Na2MoO4. In situ energy- dispersive X-ray diffraction (EDXRD) experiments revealed a correlation between particle shape, Mo content and formation pathway of the Bi2W1-xMoxO6 nanomaterials. The results are compared to related in situ studies on hierarchically structured W/Mo oxides.