A combinatorial thin,film library containing WO3 and NiO was produced using thermal co-evaporation of WO3 and Ni followed by high temperature reoxidation. Microstructure and crystallographic particularities of the WO3-NiO library investigated by SEM and XRD revealed three distinct compositional zones: A low Ni concentration zone containing crystalline WO3 and amorphous NiO, a high Ni concentration zone containing crystalline NiO and amorphous WO3 and a middle range amorphous zone connecting the extremes. Photo Electrochemical Scanning Droplet Cell Microscopy PE-SDCM was used for locally investigating the photoelectric response of the library as a function of Ni concentration. A substantial photocurrent peak was identified at 6.2 at.% Ni with values in excess of 2.5 mA cm(-2) while Ni amounts above 9 at.% resulted in a completely photoinactive thin film. XPS surface analysis indicated a surface composition different than the bulk with WO3 being present up to 45 at.% Ni in the bulk. For bulk Ni amounts above this composition, the surface contained no WO3 anymore, only an amorphous Ni sub-oxide being suggested. (C) 2014 Elsevier Ltd. All rights reserved.