o,o′‐Dihydroxyazo compounds are spectral sensitizers for the photosemiconductor properties of zinc oxide. It is shown that 1‐(2‐hydroxyphenylazo‐)‐2‐naphthol (II) forms a 1:1 Zn complex on the surface of ZnO particles if adsorbed from toluene solution. With most samples of ZnO a Langmuir type adsorption isotherm is obtained. The concentration of the dye at saturation relative to the BET surface of the ZnO particles correponds to a slightly oblique edge‐on arrangement of the dye molecules on the surfaces. In the case of ZnO pretreated with propionic acid and having taken up dye in excess of a monolayer, it can be shown by electron microscopy that tail‐like complexes (zinc: dye ratio = 1:1) are formed, and that these complexes contain zinc which was originally on the particle surfaces.