Ionization energies (IE) of [M + zH]z+ (z+) electrospray-produced polypeptides were detd. by electron ionization in a Penning cell of 4.7 and 9.4 .T Fourier transform mass spectrometers. For z = 1+ and substance P, the found IE value of 11.0+-0.4 eV is in agreement with that obtained earlier for ions generated with matrix-assisted laser desorption/ionization. For higher z, the following values were found: 11.7+-0.3 eV for 2+ of [Arg-8]-vasopressin, 11.1+-0.6 eV for 2+ of substance P, 12.2+-0.7 eV for 2+ of renin substrate, 13.3+-0.4 eV for 3+ of B-chain of insulin and 14.6+-0.6 eV for 4+ and 15.1+-0.4 eV for 5+ of melittin. It was found that 90% of existing IE data on polypeptides in the 1.0-3.5 kDa mass range are described with ? 0.5 eV uncertainty by the empirical equation IE(z) = 9.8+1.1z. The av. IE increase of 1.1 eV/charge is attributed to Coulombic repulsion. The deduced ionization energy of a neutral polypeptide mol., 9.8+-0.3 eV, is consistent with literature expectations. [on SciFinder (R)]