Zinc oxide was electrodeposited from oxygenated aqueous solutions of zinc chloride at 80 degrees C on tin oxide covered glass substrates. A new activation treatment for the substrate is established. This consists in the initial formation, in the deposition solution, of a thin metallic zinc layer (5-50 nm) converted to ZnO by in situ reoxidation. Variable densities of nucleation centers (with values approaching 10(10) cm(-2)) are formed by this treatment. This allows control of the formation of a zinc oxide layer ranging from open deposits of isolated crystallites to compact and homogeneous layers. Compact layers have high specular transmission below the band gap value (similar to 3.5 eV), whereas open films exhibit extensive light scattering. The shapes of the current-time curves during deposition are discussed in terms of nucleation and structural effects. A possible influence of the semiconducting properties of the films is pointed out.