Electrodeposition of polycrystalline semiconductor thin films is mainly used for photovoltaic applications. Most of the work concerns chalcogenide compounds like CdTe, CuInSe2, CdS... This route is also emerging for semiconductor oxide preparation. Results concerning the electrodeposition of zinc oxide layers by reduction of dissolved oxygen in presence of Zn(II) ions are presented. Epitaxial growth can be achieved in zinc chloride solutions on single crystal GaN layers. Bandgap (E-g) variations and effects on photoluminescence are observed by changing the deposition conditions and post-deposition treatments. For instance, changing the potential from -1.1 to -1.5 V versus MSE in chloride solutions increases E-g from 3.4 to 3.55 eV. Changing chloride to perchlorate anions shifts E-g from 3.52 to 3.38 eV at -1.4 V. A thermal annealing in air further decreases the E-g down to the classical value (3.27 eV). This is correlated to a decrease in ZnO lattice parameters. A concomitant variation of the photoluminescence emission in the blue is observed. The highest yield is achieved on the epitaxial layers after annealing (500 degrees C, 1 h). Specific aspects about the deposition mechanism are also discussed.