Based on MPWBIK/6-31++G(d,p) calculations, we have performed a charge and spin density analysis for the most representative structures of His delta(+) and His epsilon(+) radical cations. This analysis shows that depending on the site where ionisation takes place, fragmentation of the side-chain can occur via R+ elimination to give the m/z 81 peak in the 70 eV EI-MS spectrum, or via the most favoured RH+. elimination to give the most intense m/z 82 peak. The mechanism for RH+. elimination involves a proton-transfer from the amino group to the free N-delta 1 of the imidazole ring. (C) 2007 Elsevier B.V. All rights reserved.