Bi2WO6 has attracted considerable interest as a visible-light-driven 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)3·5H2O, K2WO4 andNa2MoO4. 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.