The electron transmission spectra of 1,3,5-tri-tert-butylbenzene, 2,4,6-tri-tert-butylpyridine, 2,4,6-tri-tertbutylphosphabenzene, and 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene have been investigated and interpreted by means of quantum chemical calculations. Scaled virtual orbital energies obtained from calculations without employing diffuse functions provide good numerical values for the vertical electron attachment energies (VAEs). B3LYP/6-311+G* VAEs, calculated as the energy difference between the anion and the neutral molecule, were in good agreement with experiment, with the molecules investigated here having VAEs of less than about 1 eV. The first anion state of phosphabenzene is predicted to lie at the edge of stability. The gradual replacement of CH units by phosphorus in pi systems results in a significant stabilization of the anionic states, which contrasts with the relative invariance of the electron donor properties. This behavior can be explained by considering that for the P=C pi system both the interaction between the atomic levels (beta) and the overlap are smaller than for the C=C pi system.