This paper examines the difference between the ionization energies of various organic molecules both in the vapour and in various crystal lattices. Photoelectric emission thresholds of organic crystals, as well as being of intrinsic interest, provide information on the energies of the ionization energy of a molecule in the gas (IG) and in a molecular crystal (Ic) measures1the calculable polarization energyPof the crystal, a key quantity in the donor-acceptor theory of organic semiconductors.