The effect of ethanol addition in ethylene laminar premixed flames on the characteristics of soot precursor nanoparticles was investigated by aerosol photoionization technique. The fifth harmonic of a Nd:YAG laser, 213 nm (5.82 eV), was used as ionization source, while a differential mobility analyzer (DMA) coupled to a Faraday cup electrometer was used for ionized particle classification and detection. Carbonaceous nanoparticles in the nucleation mode, i.e., with sizes ranging from 1 to 10 nm, show a photoionization charging efficiency clearly dependent on the percentage of ethanol used as dopant in the flame. In particular, we observed that the more ethanol was added as fuel, the lower the particle photo-charging efficiency was. This result indicates a modification in the nanoparticle chemical structure as the amount of ethanol is increased. These experimental evidences may be explained by a decrease of aromaticity of the particles and/or by the presence of oxygen bonds within the nanoparticles.