SUMMARY A straightforward flow injection method for the tensammetric determination of surfactants is described. The flow-through cell is equipped with a mercury-coated gold electrode as the working electrode. Ionic as well as nonionic surfactants can be determined in the concentration range 10~5-104 M. The calibration graph is not linear but with an approximation by a polynomial or a cubic spline function, correct results can be obtained with an accuracy of *4%. The sample rate is about 60 h-l. The determination of surfactants in aqueous solutions is of great importance both in production control and in the monitoring of, for example, sewage water. There are two types of standard method, both based on ionpair extraction [l] . Probably the most widely used is the two-phase titration procedure; the other type comprises direct photometric determinations in the organic phase. Because these methods are rather time-consuming, much attention has been given to the development of other methods during the last decade. The most important contributions are in the field of potentiometry, particularly with ion-selective electrodes [ 2-91, and tensammetry [lo--151. Recently, also automation of existing extraction procedures and subsequent photometric detection has been pursued. These attempts are either based on the continuous flow technique (AutoAnalyzer system) [ 16, 171 or on flow injection analysis [ 18, 191. The elegant phase separation by means of a permeable membrane used by Kawase and coworkers is worth mentioning. Such a phase separator will be superfluous, however, with flowthrough detectors based on potentiometry or tensammetry. Because the potentials of surfactant-sensitive electrodes are, in general, not very stable and reproducible, these electrodes are mostly used for end-point indication in titrations; they are not suitable for flow-through detectors. Detectors based on the tensammetric principle seem to be more promising in this respect. Such a flow-through detector, based on a dropping mercury electrode, has been described for applications in h.p.1.c. [14]. Although it offers the advantage of a clean surface every few seconds, it has the disadvantage that its proper functioning is sensitive to changes in flow conditions.