The mechanism of selective fluorescence quenching of alternant polycyclic aromatic,hydrocarbons (PAHs) by nitromethane has been investigated using ab initio calculations. Initial studies using 10 archetypal configurations of pyrene (an alternant PAH) and nitromethane established trends in the attractive and repulsive interactions between the two molecules. In general, the computed energy of interaction in the ground and excited states correlated with the orientation of the dipole moment of nitromethane with respect to the electrostatic potential of pyrene. In addition, many local minima were found on the potential energy surface; hence, two representative configurations (one attractive and one repulsive) were explored in detail. Calculations of the interaction of pyrene and fluoranthene (a nonalternant isomer of pyrene) with nitromethane were also conducted as a function of intermolecular separation distance. Two main routes were found for deactivation of the excited-state PAHs by nitromethane: direct energy transfer to the quencher or formation of a fluorophore-quencher ion pair. In studies of their various excited states, pyrene showed an energetically feasible path to an ion pair, whereas fluoranthene did not. In addition, simulation of PAH-solvent complexes showed that neither isomer formed energetically reasonable ion pairs with acetonitrile. Therefore, the selectivity of nitromethane for pyrene appears to be based on the relative ease with which these molecules form an ion pair and then undergo back electron transfer to regenerate the ground state.