Adsorption of three sulfonamide antimicrobials to clay minerals was investigated as a function of pH, ionic strength, and type of exchangeable cation. Sulfonamide antimicrobial adsorption exhibited pronounced pH dependence consistent with sorbate speciation and clay properties. Sulfonamide antimicrobials did not intercalate into montmorillonite, and surface charge density influenced sorption by determining adsorption domain size. Adsorption edge data were best fit to a model including terms for the cationic and uncharged species. Adsorption of uncharged sulfamethazine to montmorillonite was relatively insensitive to pH, ionic strength, and type of exchangeable cation, while that to kaolinite was highly sensitive to ionic strength. Adsorption of cationic sulfamethazine to montmorillonite exceeded that of the neutral species by 1-2 orders of magnitude, but was unimportant for kaolinite atthe pH values examined. Cation exchange appeared to contribute to sorption of cationic sulfonamide species to montmorillonite. Anionic sulfamethazine adsorption was negligible. The nature of the sulfonamide R group influenced the degree of adsorption of cationic and neutral species. Our results highlight the importance of considering sulfonamide speciation and clay surface charge density in predicting the transport of these antimicrobials.