Several water-soluble vapors of atmospheric importance adsorb at the air-water interface. This paper supplies a thermodynamic and kinetic framework for analyzing this phenomenon. As an example, temperature- and time-dependent surface-tension measurements of aqueous ammonia solutions are used to extract the interfacial binding energies and evaporation rates. The standard Gibb's energy of adsorption of vapor-phase ammonia to the water surface is -(19.1 +/- 0.5) kJ mol(-1) at 298 K; the saturated coverage is (1.2 +/- 0.2) x 10(14) molecules cm(-2). The Gibb's energy of activation for ammonia evaporation from the water surface lies in the range 13-18 kJ mol(-1) at 298 K. Ab initio calculations of the NH3-H2O and NH3-(H2O)(2) complexes have also been performed to further understand the nature of the surface-bound species. The experimental and ab initio results, taken together, suggest that ammonia is bound by a small number (two or three) of water molecules at the surface; this complex species represents a "critical cluster" which is easily transferred into the bulk solution.