Recent experiments in our laboratory have shown that the probability of gaseous HNO3 deprotonation on the surface of water is dramatically enhanced by anions. Herein, we report a quantum chemical study of how a HNO3 molecule transfers its proton upon approaching water clusters containing or not a chloride ion. We find that HNO3 always binds to the outermost water molecules both via donating and accepting hydrogen bonds, but the free energy barrier for subsequent proton transfer into the clusters is greatly reduced in the presence of Cl. As thedissociation of HNO3 embedded in water clusters is barrier less, we infer that interfacial proton transfer to water is hindered by the cost of creating a cavity for NO3. Our findings suggest that nearby anions catalyze HNO3 dissociation by pre organizing interfacial water and drawing the proton—away from the incipient [H.---NO3] close ion-pairs generated at the interface. This catalytic mechanism would operate in the 1 mM Cl range (1 Cl in 5.5 104 water molecules) covered by our experiments if weakly adsorbed HNO3 were able to explore extended surface domains before desorbing or diffusing (undissociated) into bulk water.