We have applied the mass-analyzed threshold ionization spectroscopic technique to record the cation spectra of deuterium substituted indazole and benzimidazole. The deuterium substitution causes red shifts in the ionization energies of these aza-aromatic bicyclic molecules by only a few tens wavenumbers. Comparing these data with those of deuterated aniline and indole suggests that the H/D exchange mainly takes place on the NH or CH sites of the five-membered ring. (C) 2010 Elsevier B. V. All rights reserved.