Investigations were made of the effect of pH and exchangeable cation on the adsorption of two substituted ureas and five triazines by a montmorillonite. The adsorption of the ureas was virtually independent of pH and exchangeable cation, whereas the adsorption of the triazines was influenced by both factors. It is postulated that the ureas are adsorbed by physical forces and possibly the formation of coordination complexes with exchangeable cations, while the triazines are adsorbed by a combination of these two mechanisms plus protonation and consequent ion exchange reactions, the relative importance of each process being determined by pH, exchangeable cation and the characteristics of the adsorbate molecule.Studies with mixtures of clay and organic matter show that they associate in a manner which reduces the total surface available for herbicide adsorption. It is suggested that in the soil, little of the clay mineral surface will be accessible to herbicide molecules.