RELIABLE measurement of trace species in natural waters is essential for studies of pollution or trace-element cycling, but is difficult, partly because the distribution of chemical species often changes during sampling and storage1. In situ measurements can overcome these problems, but the few measurements made previously have involved complicated systems that cannot be used routinely1,2. Here we describe a simple technique for measuring trace-metal concentrations in situ in water. The technique incorporates an ion-exchange resin separated from the solution by an ion-permeable gel membrane. Mass transport through the gel is diffusion-controlled and thus well defined, making it possible to obtain quantitative data on concentration and speciation over relatively short time periods (from one hour to several weeks). We present measurements of zinc concentrations in sea water using this technique which agree well with electrochemical measurements. In principle, our technique should be applicable to any inorganic or organic diffusing species.