We show that irreversibility of quasi-resonant bimolecular ionization in solution is due to geminate and/or bimolecular recombination of radical ions into the triplet state of neutral products. Spin conversion in the radical ion pair, which can be a limiting stage of their geminate recombination, is included in the integral encounter theory of the phenomenon. Both the triplet and charge separation quantum yields are calculated in the contact approximation and their free energy dependence is specified.