W/Mo-oxides of the hexagonal tungsten bronze (HTB) type have been investigated by X-ray absorption spectroscopy to obtain detailed insight into the substitution process of W by Mo that leads to mixed HTB frameworks. Both the morphology of the nanostructured W/Mo-HTBs as well as the oxidation state of Mo are significantly influenced through the incorporation of different alkali cations into the hexagonal channels of this open structure. A variety of complementary analytical methods, including TG, in situ and ex situ XRD, SEM, and solid-state NMR analyses, were applied to determine the thermal stability of the obtained W/Mo-HTB materials with respect to their alkali cation and NH(4)(+) contents. A strong correlation between composition and stability was found with the Rb-W/Mo-HTBs exhibiting the highest structural and morphological resistance among the series (up to 580 degrees C). The NH(3)-sensing properties of selected W/Mo-oxides in test atmospheres furthermore point to promising features of the Rb-stabilized hexagonal framework materials.