Abstract The high resolution photoelectron spectra of the three dichloroethylenes show sufficient differences for them to be readily distinguished from each other. The values for the two lowest ionization potentials are different from some determined by other methods. However the limited π-electron calculations are in reasonable agreement with our experimental results and show the need for more detailed calculations. The main changes in the spectra are due to overlapping of the non-bonding type atomic orbitals situated on the neighbouring chlorine atoms and to a lesser extent their effect on the π orbitals between the CC bond. Clearly more extensive theoretical calculations are desirable and these are being undertaken at the present time.