Co-sensitization of nanocrystalline TiO₂ with the organic dye D2 and the zinc phthalocyanine dye TT1 improves light harvesting, J_sc and efficiency of DSC devices. However, the V_oc of the co-sensitized cell is markedly inferior ( 130 mV) when compared to the reference device made with D2 only. We discuss the implications of our results with regard to selection criteria for dyes for co-sensitized DSCs.