This paper describes the influence of two polymers, fulvic acid (FA) and polyacrylic acids (PAAs) of comparable molecular mass, on the electrokinetic potential of model surfaces encountered in clay minerals : silica (SiO2), aluminum oxide (gamma-Al2O3), and gibbsite [gamma-Al(OH)(3)]. Measurements at pH 6.5 +/- 0.2 show that FA and PAAs modifiy the potentials of aluminum oxide and gibbsite, but leave the silica surface unchanged. A chlorite clay mineral (ripidolite), milled to increase the numbers of oxy-hydroxy groups at newly created surfaces, was exposed to FA and PAAs, carriers of carboxylic groups, to study their influence on electrokinetic potential. The key to the interaction is that polymers hold metal ions (Al, Mg, Fe) in the edge surfaces, while siloxane groups show limited interaction or none at all. The results offer an explanation of why clay mineral particles are always negatively charged in natural waters.