The dye-sensitized solar cell (DSC) challenges conventional photovoltaics with its potential for low-cost prodn. and its flexibility in terms of color and design. Transient absorption spectroscopy is widely used to unravel the working mechanism of DSCs. A unexplained feature obsd. in these studies is an apparent bleach of the ground-state absorption of the dye, under conditions where the dye is in the ground state. Here, this feature can be attributed to a change of the local elec. field affecting the absorption spectrum of the dye, an effect related to the Stark effect. The authors present a method for measuring the effect of an externally applied elec. field on the absorption of dye monolayers adsorbed on flat TiO2 substrates. The measured signal has the shape of the 1st deriv. of the absorption spectra of the dyes and reverses sign along with the reversion of the direction of the change in dipole moment upon excitation relative to the TiO2 surface. A similar signal is obsd. in photoinduced absorption spectra of dye-sensitized TiO2 electrodes under solar cell conditions, demonstrating that the elec. field across the dye mols. changes upon illumination. This result has important implications for the anal. of transient absorption spectra of DSCs and other mol. optoelectronic devices and challenges the interpretation of many previously published results.