The influence of cyclic pi-conjugation and interannular rotation on the vertical ionization energies and energy gaps of phosphole oligomers has theoretically been investigated which provided some useful structure-property relationships. While vertical ionization energies were obtained from negative HOMO-energies using the Hartree-Fock method, energy gaps were calculated using a DFT/hybrid method (B3LYP). Combination of geometric (JULG and d(Cx)) and magnetic (NICS) criteria gives a qualitative indication about the competition between the cyclic- and the carbon backbone pi-conjugation. Results of this preliminary theoretical study suggest that phosphole oligomers have interesting tunable electronic properties. (C) 2002 Elsevier Science B.V. All rights reserved.