The electronic structure of 1-nitrosopyrrolidine has been investigated by HeI and HeII LTV photoelectron spectroscopy and DFT/ CCSD/OVGF calculations. Even high-levels of theory and large basis sets failed to reproduce the measured molecular structure or vertical ionization energies in a consistent manner. The unexpectedly large intensity enhancement of the band corresponding to HOMO ionization was observed in the HeII photoclectron spectrum of 1-nitro sopyrrolidine. Our results indicate that the HOMO and SHOMO ionizations in N-nitrosamines have a and pi characters, respectively rather than the reverse as was previously thought. We relate the electronic structure to the genotoxic, carcinogenic properties of N-nitroso compounds. (C) 2007 Elsevier B.V. All rights reserved.