Molecular ions often play a very important role in plasmas. The electron energy distribution function (eedf) and density (n(e)) are influenced by the reactions of molecular ions with electrons. We bring these aspects into focus by studying successively the following situations: We show that the dissociative recombination of Ar-2(+) allows to understand the measured characteristics of an argon supersonic plasma flow where the electron density is low. Afterwards, we show the dominating role of NO+ in the chemistry of a space vehicle atmospheric re-entry air plasma. Finally, by using the Boltzmann equation in order to show the influence of molecular ions such as NO+ in air plasmas on the eedf, we comment on the common assumption of Maxwell-Boltzmann equilibrium for this distribution.