The highest occupied molecular orbital (HOMO) of cyclopentene (C5H8) has been studied by a binary (e, 2e) electron momentum spectroscopy, at an impact energy of 1200 eV plus the binding energy and using symmetric non-coplanar kinematics. The first measurements of the complete valence shell binding energy spectra and the HOMO momentum profile are reported. The experimental momentum profile of HOMO is compared with the theoretical momentum profiles calculated using Hartree-Fock and density functional theory method with various basis sets. The EMS experimental result is well described by the calculations. (C) 2004 Elsevier B.V. All rights reserved.