Quantum chemical techniques, including molecular orbital and density functional theory methods, have been applied to investigate the electronic and thermochemical properties of mono-fluorinated phospholes. More specifically, we investigated in a systematic way the influence of F-substitution at three different ring positions on the ionization energy, the frontier orbital energy gap, the site of protonation and the basicity of phospholes. The most remarkable effect is that following F-substitution at P, the energy gap is reduced by nearly 1 eV. A similar decrease could also be expected on the electron affinities of these F-phospholes. (C) 2003 Elsevier B.V. All rights reserved.