Formation of iodine from air-saturated KI solution under natural sunlight (950 +/- 25 W m(-2)) with nanocrystals of TiO2 in different phase compositions, ZnO and SnO2, characterized by powder XRD, UV-visible diffuse reflectance and impedance spectroscopies, increases by 3-9 fold on impressing a cathodic bias of -0.2 V (vs NHE) to the semiconductors. The iodine generation essentially requires light, the semiconductor and dissolved oxygen. An anodic bias of +0.2 V (vs NHE) to the illuminated semiconductors fails to oxidize iodide ion from air-saturated KI solution. Under cathodic bias, mixtures of ZnO-TiO2 rutile, SnO2-TiO2 rutile and ZnO-SnO2 nanocrystals show higher photocatalytic activity, pointing to an improved charge separation in the oxide mixtures. The mechanism and the energetics of photocatalysis under cathodic bias are discussed. Also, the photocatalytic activities of the semiconductor nanocrystals are analyzed in terms of their optical, electrical and crystal properties. (C) 2010 Elsevier B.V. All rights reserved.