The adsorption and desorption of 2-chloro-4-ethylamino-6-isopropyl-amino-s-triazine (atrazine) as affected by pH, temperature, and cencentration of atrazine were studied on Drummer clay loam soil and a humic acid isolated from Leonardite. The adsorption of atrazine on the soil increased as pH decreased, but it was affected only slightly by temperature and cencentration of atrazine. Desorption of atrazine was greater with increasing temperature and pH. The adsorption of atrazine on the soil was reversible and desorption was nearly complete. Adsorption of atrazine on the humic acid increased as pH decreased and temperature increased, and there was very little desorption of atrazine from the humic acid.