A technique has been developed for directly measuring the amount of surfactant, which is suitably labelled with a ¹⁴C atom, adsorbed at an oil-water interface. The technique entails using an oil-soluble, non-surface-active scintillant and a photomultiplier. The number of pulses detected is proportional to the ¹⁴C surfactant concentration at the interface and after suitable calibration this can be transformed into the surface concentration. As an example, the technique is used to measure the adsorption isotherm for cetyl-1-¹⁴C-trimethylammonium bromide at the toluene-water interface.