Compact nickel oxide (NiO) thin films were prepared on various substrates via a simple spray pyrolysis technique. Morphological and structural characterization indicates that these NiO films are very uniform in thickness (similar to 100 nm) and possess the bunsenite crystal structure. Optical measurements show that the NiO films are highly transparent with a band gap of 3.70 +/- 0.05 eV. Mott-Schottky plots obtained from electrochemical impedance spectroscopy measurements reveal that the as-deposited NiO on fluorine-doped tin oxide (FTO) glass behaves as a p-type semiconductor. The flat band potential of NiO was estimated to be similar to 0.36 V (vs. NHE) in 0.10 M tetrabutylammonium perchlorate/acetonitrile electrolytes. Cyclic voltammetric measurements of the NiO films on FTO in various redox electrolytes show that electrochemical reactions proceed in the accumulation region but are completely inhibited in the depletion region, indicating the NiO films effectively block the FTO substrate. Using these NiO blocking layers, a CdS-sensitized mesoscopic NiO photocathode operating in a polysulfide electrolyte is unambiguously demonstrated for the first time. It is anticipated that NiO thin films synthesized by spray pyrolysis could find important applications as stable and transparent electron barrier layers for various optoelectronic devices. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3590742] All rights reserved.