To clarify the relationships between the performance of dye-sensitized solar cells and their internal resistances, we applied ac impedance spectroscopy to the analysis of the cells. Impedance and <mml:math display="inline"><mml:mi>I</mml:mi><mml:mo>­</mml:mo><mml:mi>V</mml:mi></mml:math>I­V measurements of solar cells were carried out for various cell compositions and measurement conditions such as different kinds of conductive glass substrates, two kinds of counter electrodes, calcination temperature, applied bias, and distance between electrodes; in order to separate internal resistances which occur at different interfaces. Impedance spectra of dye-sensitized solar cells showed that their internal resistance consisted of at least five components. It was found that the resistance of a conductive glass substrate appeared as ohmic resistance, and that electron transfer at F-doped <mml:math display="inline"><mml:msub><mml:mi mathvariant="normal">SnO</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:math>SnO2 (FTO)/ <mml:math display="inline"><mml:msub><mml:mi mathvariant="normal">TiO</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:mo>,</mml:mo></mml:math>TiO2, <mml:math display="inline"><mml:msub><mml:mi mathvariant="normal">TiO</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:mo>/</mml:mo><mml:msub><mml:mi mathvariant="normal">TiO</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:mo>,</mml:mo></mml:math>TiO2/TiO2, Pt/electrolyte, and <mml:math display="inline"><mml:msub><mml:mi mathvariant="normal">TiO</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:math>TiO2 /electrolyte interfaces was identified as the internal resistances determinative to the performance of dye-sensitized solar cells. Internal resistances at <mml:math display="inline"><mml:mo>FTO</mml:mo><mml:mo>/</mml:mo><mml:msub><mml:mi mathvariant="normal">TiO</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:mo>,</mml:mo></mml:math>FTO/TiO2, Pt/electrolyte and the resistance of conductive glass affected the fill factor. Internal resistances at <mml:math display="inline"><mml:msub><mml:mi mathvariant="normal">TiO</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:mo>/</mml:mo><mml:msub><mml:mi mathvariant="normal">TiO</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:math>TiO2/TiO2 and <mml:math display="inline"><mml:msub><mml:mi mathvariant="normal">TiO</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:math>TiO2 /electrolyte mainly affected the photocurrent. Particularly, the <mml:math display="inline"><mml:msub><mml:mi mathvariant="normal">TiO</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:math>TiO2 /electrolyte was significant as the interface controlling photocurrent of the dye-sensitized solar cell. The impedance stemming from the diffusion of the electrolyte had little effect on the performance of the cells. © 2005 The Electrochemical Society. All rights reserved.