Acetonitrile solutions of Cu(tmby)22+ exhibit aging-dependent changes of UV–Vis spectra, which were attributed to partial reduction to Cu(tmby)2+. This effect is boosted by the addition of imidazole bases, i.e., N-methyl-benzimidazole (NMB) or 5‑chloro-1-ethyl-2-methyl-imidazole (CEMI). These bases have only small effect on the diffusion coefficients of Cu(tmby)22+, but downshift the redox potential of Cu(tmby)22+/+ by ca. 0.1–0.2 V. The addition of CEMI to the electrolyte solution decreases the flatband potential of the TiO2 anatase (101) face. The corresponding position of conduction band minimum is upshifted by 0.19 or 0.26 eV, depending on the counterion type, Li+ or Na+, respectively. The formal redox potentials of Cu(tmby)22+/+ in the CEMI-modified electrolyte solution, interrelated with the respective flatband potentials in TiO2, provide a refined estimate of the maximum accessible open-circuit photovoltage of DSSC.