Abstract The He(II) photoelectron spectra of acrolein, propynal, propynol and propiolic acid are interpreted with the aid of ab initio many-body Green's function calculations in the outer and inner valence region. Koopmans' approximation fails to supply the correct ordering of electronic states in the radical cation if the lone pair orbital on the oxygen atom is of a′ symmetry. In the case of propynal the ordering of two ionic states o f equal symmetry is interchanged with respect to the Koopmans' orderin. A strong mixing of hole configurations occurs for outer valence states of propynol and propiolic acid. The implications of this mixing are briefly discussed.