The SCF-LCAO-MO wavefunction for the positive ion of pyridine obtained by ionization in the π MO of a2 symmetry is reported. It is shown that the use of orbital energies in predicting ionization potentials yields correct results only in the case of cancellation of terms. The details of the cancellation are analyzed. The π-electron ionization brings about a large charge rearrangement in the a electrons. The loss of a π charge is balanced by donation of σ electrons from the hydrogen atoms to the ring. The ring atoms are somewhat less negative in the ion than in the neutral molecule; the hydrogen sites are substantially more positive. The over-all electronic rearrangement following the π-electron ionization is about 1.3 σ electrons and 0.2 π electrons. This rearrangement is done at the expense of about 1 eV.