Electrochromic (EC) NiO(z) and WO(y) thin films were prepared by sputtering and were used in a feasibility study aimed at investigating mixtures of these two oxides. The object was to identify a suitable electrolyte, compatible with both NiO(z) and WO(y). To that end we carried out cyclic voltammetry (CV) in potassium hydroxide (KOH), propionic acid, and lithium perchlorate in propylene carbonate (Li-PC). WO(y) could be coloured in propionic acid and Li-PC, while NiO(z) could be coloured only in KOH. Both films showed best stability in Li-PC, which hence is well suited for further studies of mixed NiO(z) and WO(y). (C) 2009 Elsevier B.V. All rights reserved.