Charge sepn. in sintered colloidal TiO2 film electrodes was studied by action spectra in SCN-/(SCN)2- the UV region. Electrodes with different film thicknesses were studied. Current generation is most efficient close to the back contact. Monochromatic quantum efficiency of ≤77% and optimum overall quantum efficiency were attained at a colloidal film thickness of ∼4 μm. Illumination through the back side contact gives a theor. quantum efficiency of 100% to a depth of 0.5 μm from the back contact. The origin of the unexpectedly high current generation in the colloidal films was studied.