This work focuses on the development and application of the UV diffuse reflectance-laser flash photolysis technique to directly study the kinetics of reactions occurring at the gas-liquid-phase boundary. The reaction of Cl-2(.-) radical anion with ethanol was chosen to directly "probe" the reaction kinetics at the air-water surface. The reaction rates at the surface are shown to be more rapid than in the bulk liquid. Direct kinetic evidence is provided that the reaction of Cl-2(.-) radical anion with ethanol is at least 2 times faster at the surface than in the bulk. The rate coefficient for the surface reaction Cl-2(.-) + ethanol is found to be (4.45 +/-0.80) x 10(5) M-1 s(-1). For comparison, the rate coefficient for the reaction of Cl-2(.-) with ethanol in the aqueous phase is found to be (1.77 +/- 0.34) x 10(5) M-1 s(-1). The uncertainties in the above expressions are 2sigma and represent precision only. The effective rate coefficient for the aqueous-phase reaction Cl-2(.-) + ethanol is found to be consistent with what has been reported. Therefore, the chemistry at the interface differs from that of the gas phase or liquid phase. The nature of interfacial reactions and their atmospheric implications are discussed.