The dissociation mechanism of excited CH2X2 (X = Cl, Br) was investigated using charge-inversion mass spectrometry, in which positive ions collide with an alkali metal target to generate neutral fragments, and the negative ions formed from the neutral fragments are mass analyzed. Different relative abundances of the negative ions were observed in the charge- inversion spectra for CH2Cl2 and CH2Br2. The kinetic energy release values calculated from analysis of the peak associated with CHCl2- in the charge-inversion mass spectrum of the parent CH2Cl2+ ion indicate that the excited CH2Cl2 formed by neutralization dissociates spontaneously into CHCl2 + H. (c) 2008 Elsevier B.V. All rights reserved.