Cosensitized plastic-substrate dye-sensitized solar cells (PDSCs) with Black dye and a pyridine-anchor organic dye (NI5 or YNI-2) have been prepared to improve conversion efficiency. Detailed analysis of light-harvesting efficiency spectra and ATR-IR spectra of the cosensitized TiO2 photoelectrodes revealed that the amount of pyridine anchor dye, which adsorbs to the Lewis acid sites of the TiO2 surface and works as a sensitizer, increased with prolonging the immersion time of the Black-dye-adsorbed TiO2 photoelectrodes into the dye-adsorption solvent of pyridine anchor dye up to 60 mm. In addition, these measurements also revealed that the dye aggregation between Black dye and pyridine anchor dye occurred just after immersion, even though the amount of pyridine anchor dye which forms dye aggregation with Black dye is small when the immersion time is shorter than 30 mm The conversion efficiency of the Black-dye-based PDSC was improved by cosensitization with pyridine anchor dyes up tol 30 mm immersion, and it decreased largely when the immersion time is extended to 60 mm. A conversion efficiency of 8.3% (AM 1.5, 100 mW cm(-2)), which is the highest conversion efficiency in the PDSCs, could be achieved in the cosensitized PDSC with Black dye and NI5 prepared under the optimized conditions..