The variation of photoelectrochemical behavior in the early stage of a Ru(II)-dye-sensitized TiO2 solar cell with the addition of carboxylic acid in I-/I-3(-) acetonitrile solution is described. In the presence of carboxylic acid, photocurrent-voltage curves exhibit dramatic enhancement in stability, increase in photocurrent, and decrease in photovoltage compared with those in the absence of carboxylic acid. The changes were studied by measurements of flatband potential, UV-visible absorption and reflectance spectra, and ac impedance spectra. The measurements revealed that adsorption of carboxylic acid on the TiO2 surface plays a major role in variations of the photocurrent-voltage characteristics.