The transport and back reaction of electrons in dye sensitised nanocrystalline solar cells (DSNC) has been studied by frequency resolved optical perturbation techniques. Intensity modulated photocurrent spectroscopy (IMPS) has been used to obtain values of the electron diffusion coefficient, D n, as a function of illumination intensity. It was found that D n increased with intensity (D n∝I −0.5). Intensity-modulated photovoltage spectroscopy (IMVS) has been used to measure the electron lifetime, τ n, which is determined by the rate of back reaction with I 3 − ions in the electrolyte. It was found that τ n decreased with light intensity (τ n∝I −0.5). The electron diffusion length, L n=(D n τ n)1/2, is therefore only weakly dependent on light intensity. The values of L n were used to calculate the theoretical IPCE of the cell. Experimental measurements confirmed the prediction that the IPCE should remain almost constant over five orders of magnitude of light intensity. Possible reasons for the opposite trends in D n and τ n are discussed and related to the fundamental processes taking place in the DSNC.