Molecular beam scattering experiments are used to identify a submicrosecond, near-interfacial DCl --> HCl exchange pathway following collisions of DCl with pure and NaOH-doped glycerol. We find that similar to7% of the thermally equilibrated DCl molecules undergo D --> H exchange at or just below the surface of glycerol and then desorb immediately as HCl before they enter the bulk liquid. The data are consistent with exchange occurring within a near-interfacial cage of glycerol molecules, either through an ionic or neutral reaction between the DCl molecule and the surrounding OH groups of glycerol. An additional similar to20% of the thermalized DCl molecules desorb immediately before proton exchange can occur, whereas the remaining similar to73% of the DCl molecules dissociate in the interfacial or deeper regions and dissolve as ions within bulk glycerol for long times. The existence of a rapid D --> H exchange and desorption pathway demonstrates that reactions between a gaseous acid and a protic solvent can be confined to the near-interfacial region of the solvent even when bulk solvation of the ionized acid is very favorable.