Collisional activation of protonated 3-halogenopyrazoles (X-Pz, X = Cl, Br and I) in the high or low translational energy regime induced an intense loss of X giving C3H4N2.+ radical cations whose structure depends on the nature of the halogen. Protonated 3-1-Pz generated thus ionized pyrazole a, whereas protonated 3-Cl-Pz was a precursor of an isomeric species ascribed to a dehydropyrazolium distonic structure b. A mixture Of C3H4N2.+ ions was formed in protonated 3-Br-Pz. B3LYP/6-31++G(d,p) computations confirmed a regiospecific N-2-protonation, and a low energy content of the distonic ions b or c (50 kJ mol(-1) above a and lying in deep energy wells). Two competitive C-H and C-X bond cleavages were invoked to explain the contrasting behaviour of various protonated X-Pz under dehalogenation conditions. (C) 2001 Elsevier Science B.V. All rights reserved.