Clay minerals have long attracted the attention of colloid scientists, This paper considers, specifically, their important role in the transport of various contaminants from land to sea, e.g., metal ions and organic detrital and man-made material in watercourses. Advances in experimental techniques have enabled precise characterization of clays and then electrokinetic experiments at high electrolyte concentrations, such as in seawater. Three of the most important clay minerals encountered in suspended matter in natural waters, montmorillonite, illite, and chlorite, were prepared in a very pure state. Electrokinetic experiments were done in pure aqueous single and complex electrolyte solutions and in solutions in which natural organic matter was simulated using a humic substance, fulvic acid, of defined provenance and properties, typical of riverine waters. An isoelectric point was found at pH 5.0 +/- 0.2 for chlorite; none were found for illite and montmorillonite. Only Ca2+ showed a charging effect on chlorite, indeed a reversal of sign from negative to positive at 1 x 10(-3) mol dm(-3). Addition of fulvic acid affected only chlorite, illite less, and Na montmorillonite not at all.