Electrochemical studies on 18 samples from six weathered soils from tropical north Queensland showed that they contain colloids of largely the 'constant surface potential' type, i.e, their surface charge is controlled by the pH and ionic strength of the ambient medium. The point of zero charge (PZC), as well as the net charge at the soil pH and pH 7.0 at two electrolyte concentrations, were established. The implications with respect to cation exchange capacity, base saturation, buffer capacity, and water dispersible clay, and the importance of these properties to soil classification and agricultural management, are discussed.