Measurements of the electroluminescence of dye-sensitized solar cells are presented for the first time. Forward biasing of a dye-sensitized solar cell leads to electron injection into the dye which is adsorbed on a nanocrystalline TiO2 film. Application of the generalized Planck equation yields Delta mu(D), the difference of chemical potentials of dye molecules in an excited state and in the ground state, which are involved in the optical transition. We find that Delta mu(D) is equal to the voltage between the contacts of the cell, after the latter is corrected for a voltage drop over a series resistance.