The photoionization curves and the threshold energies for the molecules vinylchloride, vinylfluoride, 1,1-difluoroethylene and their abundant fragment ions have been measured with synchrotron radiation in the photon energy range from 10 - 23 eV. Appearance potentials were used to calculate heats of formation, bond energies and ionization potentials. The structure of ion efficiency curves is discussed in terms of different ionization processes.