By means of two experimental techniques, the sensitization of the conductivity and the discharge both of ZnO single crystals and zinc oxide-resin layers with rhodamin B, eosin, and methylene blue have been studied. Stimulated by current-voltage and capacitance measurements with ZnO single crystals in a suitable electrochemical cell, the sensitization was investigated of anodically polarized ZnO crystals by dyes soluted in the electrolyte of the electrochemical cell: + In || ZnO single crystal || electrolyte + dye || platinum -. A strong increase of the anodic current was obtained by illumination with the corresponding light through the crystal from the backside. In addition to the photovoltage, the photocurrent of the anodically polarized ZnO was measured as a function of the dye concentration in the electrolyte at a constant voltage (2 V). To get more information on the mechanism, these experiments were completed by discharge measurements both of ZnO-resin layers and ZnO single crystals charged under a corona with a dc voltage. The dyed zinc oxide was illuminated under similar conditions. At approximately the same surface concentration of the corresponding dye (as mentioned for the electrochemical experiments) and at the same flux of photons, the decay of the surface potential both of ZnO single crystal and a ZnO-resin layer with the time was measured under illumination with light of various wavelengths. Some experiments on the influence of a second dye in the electrolyte on the extent of sensitization by the first dye are discussed.