An electrochemical cell is described which enables simultaneous optical transmission measurements and X-ray diffraction studies. This combination of techniques is particularly interesting in the field of electrochromics, since direct correlation of optical and structural data is possible. The electrode material is applied on a thin-film polymer window which is both optically transparent and X-ray transparent. The importance of some ensuing complications in the X-ray diffraction analysis is discussed. The cell design provides a uniform current distribution over the entire electrode surface area. Preliminary experiments on Li intercalation in 0.5 mu m sol-gel V2O5 films and hydrogen incorporation in 0.2 mu m yttrium films demonstrate the possibilities of the new design.