The photoelectrochemical effects of a series of phenylalkanoic acids as co-adsorbents along with a ruthenium sensitizer, cis-diisothiocyanato-bis(2,2'-bipyridyl-4,4'-dicarboxylato) ruthenium(II) bis-tetrabutylammonium dye (N719), were investigated in dye-sensitized solar cells (DSSCs). All the co-adsorbents used in this study enhanced the short circuit current density (Jsc) resulting from the increased electron injection efficiency due to the significant reduction of dye-aggregation, and this enhancement increased with increasing chain-length of their hydrophobic alkyl groups. They suppressed interfacial back electron transfer rate from the conduction band of TiO2 to triiodide (I3-) in electrolyte without shifting the quasi-Fermi level of the TiO2 in general, and induced a longer recombination lifetime (tau(r)). This corresponded to the enhancement of the open circuit voltage (Voc) of the cells by up to ca. 20 mV compared to that with no co-adsorbents. On the contrary, the photovoltage decreased by ca. 10 mV when 2-phenylethanoic acid was used as a co-adsorbent, which was attributed to the negative shift of the quasi-Fermi level of the TiO2 nanoparticles.