The adsorption of 13 related s-triazine compounds on montmorillonite clay as influenced by pH and molecular structure was investigated. Adsorption isotherms for prometone on montmorillonite were determined at six pH levels. The maximum adsorption of all the compounds on montmorillonite was found to occur at a pH in the vicinity of the pKA value of each compound. Substituent groups on the triazine ring were found to be related to the amount of adsorption which resulted. Mechanisms of adsorption were postulated and discussed.