This paper deals with the Penning ionization of the CO, N₂ and C₂H₂ isoelectronic molecules with the Ne* metastable atoms. All the experimental electron- and mass-spectroscopy results known now are presented. The origin of the contrasting results observed, (a weak ionization cross section and an extensive vibrational population of the residual ion molecules for the CO and N₂ in the one hand, and a large ionization cross section and a Franck- Condon vibrational population for C₂H₂ in the other hand) is discussed, then explained in the light of the photoionization results using the NeIα radiation. We show that such a divergence is consistent with the role played by the additional ionization process, i.e. an excitation transfer via an autoionizing Rydberg state in the case of CO and N₂ whereas it is via a π* shape resonance for C₂H₂.