Cohen and Fano derived in their famous paper of 1966 a formula which describes the partial cross section oscillations of the ionization probability of mirror symmetric systems such as diatomic homonuclear molecules but also of larger system such as fullerenes. The general prediction of oscillating partial cross sections has been proven since that for different excitation schemes like ion, electron and photon impact. The specific threshold behavior of the Cohen-Fano formula, however, has been experimentally never proven. We present new measurements on this intriguing problem for N₂ and C₆₀.