Sulfonamide antimicrobial agents are widely used in human medicine and animal husbandry and enter soil environments through effluent irrigation and application of manure as soil amendment. The interaction of sulfonamide antimicrobials with soil constituents influences their mobility and bioavailability. In this study, we examined the sorption and desorption of three representative sulfonamide antimicrobials (viz. sulfamethazine, sulfamethoxazole, sulfapyridine) to reference clay minerals with and without coatings of humic acids (Elloit Soil humic acid and Summit Hill humic acid). Sorption of sulfonamide antimicrobials to humic-clay complexes was linear (n = 0.94 – 0.97) over the investigated concentration range. Increased humic acid loading enhanced sulfonamide sorption and slightly decreased sorption linearity. The effect of humic acid loading and sorbent composition on sorption-desorption hysteresis will be discussed.