IR spectra of isolated Ar complexes of the phenol, para-fluorophenol, and para-chlorophenol cations (Ph+-Ar, p-FPh+-Ar, p-ClPh+-Ar) are analyzed in the vicinity of the O-H stretch fundamental, nu(1). Two isomers of each dimer are produced in the electron impact ion source and identified by their characteristic nu(1) frequency, namely the H-bound global minimum and the less stable pi-bound local minimum. The analysis of the nu(1) frequencies indicates that the strength of the O-H bond in p-XPh+ increases by halogen substitution in the order X = H < F < Cl. Quantum chemical calculations support the interpretation of the experimental data. (C) 2003 Elsevier Science B.V. All rights reserved.