High-level quantum chemical calculations on the [AlHF](+) potential energy surface identify HAlF+ and AlFH+, two isoenergetic isomers (E-rel(HAlF+) congruent to 0 +/- 5 U mol(-1)) separated by a large barrier (E-rel(TS) = 245 kJ mol(-1)). While barriers of similar magnitude obstruct [AlHCl](+) and [AlHBr](+) isomerization also, in the latter two systems the proton affinity of AlCl and AlBr is 25-40 kJ mol(-1) larger at Al than at the halogen atom. The preference for protonation of the less electronegative metal atom, rather than the halogen atom, appears counterintuitive but can be rationalized through analysis of the bonding modes in AlHX+ isomers. (C) 2003 Elsevier B.V. All rights reserved.