The surface-sensitive spectroscopic technique of optical second harmonic generation (SHG) is employed to study the adsorption and photochemistry of surfactant molecules at the water/1,2-dichloroethane (DCE) interface. Resonant SHG measurements at 730 nm are used to monitor the adsorption of the azobenzene surfactant 4-[[(dodecyloxy)benz-4-yl]azo]benzoic acid (DBA) from DCE solution to the interface at an aqueous pH of 8 or greater. The concentration and pH dependence of the resonant SHG from the adsorbed monolayer indicates that the DBA exists in its anionic (carboxylate) form at the interface. In a series of combined photochemical/SHG experiments the trans/cis photoisomerization of the adsorbed DBA anions is examined. SHG measurements demonstrate that illumination of the surface with light at 365 nm converts the adsorbed trans-DBA molecules to the cis isomer. The cis-DBA anion is found to be unusually stable at the liquid/liquid interface, and the entire photoactive monolayer (total area 4 cm(2)) can be photochemically converted in approximately 450 s with an illumination spot of 0.12 cm(2). This relatively short conversion time is attributed to surface-tension-induced convection effects that occur during the photochemical conversion of the monolayer. Illumination of the cis-DBA anions with light at 440 nm reconverts the entire surfactant monolayer back to the trans-DBA form in the same time frame.