Electron accumulation in illuminated dye-sensitized nanocrystalline TiO2 photovoltaic cells has been detected by measuring the photoinduced change in transmission at 940 nm. The sensitivity of the transmission measurement was enhanced by using intensity-modulated visible light (514 nm) to excite the photosensitizer dye. The intensity modulation was superimposed on a larger de illumination level in order to allow linearization of the intensity-modulated transmission, photovoltage, and photocurrent responses. The dye-sensitized photovoltaic cell was also characterized by measuring its frequency-dependent impedance under steady illumination. Comparison of the normalized modulated transmittance Delta T/T at open circuit with the corresponding modulated photovoltage response was used to derive the optical absorption cross section of electrons in the cell (sigma(n) similar or equal to 10(-17) cm(2) at 940 nm). The net electron injection efficiency was found to be 1 at short circuit but only 0.3 at open circuit, indicating that under strong accumulation the reaction of electrons from the TiO2 with the oxidized dye competes with dye regeneration by iodide ions, Delta T/T was also measured under short-circuit conditions, and the results showed that the decrease in electron density is less than that predicted for the case of trap free electron transport. Complex plane plots of the frequency-dependent Delta T/T response were found to be semicircular for both open-circuit and short-circuit conditions. The minima of the semicircles occurred at the same frequencies in the two cases, indicating that the majority of the detected electrons are trapped and that their concentration relaxes predominantly via back reaction with I-3(-).