ZnO based dye-sensitized solar cells have been studied using N719 and Z-907 as sensitizing dyes, with and without adding water to the dye soln. The solar cells have been characterized using photoelec. measurements and the interface between the dye and the ZnO surface has been studied using photoelectron spectroscopy. It was shown that water in the dye soln. greatly reduces surface dye aggregation and thereby enhances the solar cell performance for N719. For Z-907 where no sign of dye aggregation could be found, the presence of water had minor effect on the surface structure and solar cell performance.