Protonated 2-Cl and 2-Br-pyridines undergo facile dehalogenation upon high energy (8 keV range) collisional activation provided the target is NO, or even better O-2, instead of He or Ar. In the low energy regime (20-30 eV range), debromination occurs more readily than dechlorination, but the peculiar behaviour of O-2 in favouring an X-loss over a HX-elimination, is no longer detected (using a MS3 instrument). B3LYP/6-31G(d) calculations on X-pyridines with X = F, Cl and Br on positions 2, 3 and 4, suggest that, if a protonated pyridine is formed in the lower-lying triplet state following strong interaction with O-2 or NO having higher spin, the loss of Cl or Pr becomes almost spontaneous (F-loss is more difficult). Proton affinity PA(pyridine) = 940 +/- 15 kJ/mol, is decreased by 5-15 kJ/mol upon halogenation.