Nanostructured ZnO films consisting of 150-nm sized particles were dye-sensitized with different ruthenium complexes. Incident photon-to-current conversion efficiencies (IPCE) were found to be dependent on dye concentration and on residence time of the ZnO film in the dye solution. Results obtained using infrared and Raman spectroscopies together with traditional two-electrode measurements using a sandwich-type cell gave evidence of dye agglomeration in the nanostructured ZnO film. Possible reasons for the agglomeration process in ZnO films are discussed.