At 20 degreesC, ammonia is adsorbed in the aqueous interface in a limiting monolayer, unlike less soluble gases that exhibit adsorption moving smoothly from monolayer to multiple layers. The limiting monolayer appears at x(NH3) greater than or equal to 0.54. The effective diameter of NH3 in this monolayer is 0.293 nm, indicating that ammonia in the interfacial phase is NH3, not NH3.H2O. In ammonia concentrations where interfacial water OH groups are available, these findings are consistent with published reports showing orientation of the hydrogen-bonded complex to leave the water moiety in the liquid phase adjacent to the interface.