High resolution valence photoelectron spectrum of L-proline, up to 20 eV, was recorded using He-I radiation (21.218 eV) as ionization source in the gas phase. The thermal stability of the sample was checked by recording its mass spectrum at different temperatures to select the proper temperature for evaporating the sample without degradation. Symmetry adapted cluster/configuration interaction (SAC-CI) method based on the single and double excitation (SD-R) operators and natural bonding orbital (NBO) calculations were employed to calculate and assign the photoelectron bands of the four most stable conformers of gaseous L-proline, respectively. (C) 2013 Elsevier B.V. All rights reserved.