The momentum distributions of valence orbitals of pyrimidine were measured at various impact energies. The observed distributions were compared with HF, DFT-B3LYP, and OVGF calculations. The HOMO was unambiguously assigned to the 7b(2) symmetry. It was found that the pole strength of the outer valence orbital 2b(1) was 0.8. Moreover, the present study indicated that we should be very cautious in using the (e, 2e) reaction to investigate the molecular conformational population by comparing the PWIA theoretical results with the experimental results at a single impact energy. (C) 2009 Elsevier B. V. All rights reserved.