Assignment of the photoelectron spectrum of methanesulfenic acid, H3CSOH, is facilitated by ab initio electron propagator calculations. Close agreement exists between experimental peak positions and predictions at the NR2 level of theory for the first three ionization energies. A fourth state, which may be obscured by impurities generated in the thermolytic synthesis of evanescent H3CSOH molecules, is predicted. A nearby correlation state has also been discovered. Two higher ionization energies predicted by NR2 theory are in close agreement with a feature identified in the spectrum. Finally, a previously unassigned peak at higher energy has been explained on the basis of-the NR2 calculations. Dyson orbitals corresponding to each ionization energy are analyzed in terms of their uncorrelated, Hartree-Fock counterparts and their atomic constituents.