Abstract Gaseous and solid-state ionization potentials of several derivatives of tetraselenafulvalene (TSF) and tetrathiafulvalene (TTF) have been determined from ultraviolet photoelectron spectroscopy. The first ionization potentials of the molecules in the gaseous state are appraised in terms of their molecular structures and the substitution effects. The characteristics in the electronic structure of these molecules as donors forming conductive salts with various acceptors are discussed. The polarization energies, obtained as the difference of the threshold ionization potentials for both states of these compounds, ≈1.4 eV, were found to be a little smaller than the nearly constant value for polycyclic aromatic hydrocarbons, ≈1.7 eV. As the value of the polarization energy is determined mainly by the molecular polarizability and by the molecular packing density in the solid, the molecular polarizabilities of these compounds were estimated from the observed polarization energies, using a simplified relation. Such values are also useful in characterizing electronic states of the solids containing them.