The deuterium isotope effect on the fluorescence lifetime of contact ions pairs (CIP) composed of 9,10-dicyanoanthracene and weak aromatic electron donors in acetonitrile has been investigated. For all pairs studied, an increase of the emission lifetime with increasing deuterium substitution on both the electron acceptor and the electron donor was observed. The free ion yield shows the same variation upon deuteration. It is concluded that the efficiency of free ions formation is essentially determined by the competition, within the CIP, between charge recombination and dissociation into free ions and that loose ion pairs do not play a significant role.