Recently, public waterways have often been contaminated by numerous agricultural chemicals eluted from agricultural areas and golf links. In practice, however, the surface waters are complicated with coexisting agricultural chemicals and humic substances. Activated carbon adsorption is considered one of the solutions for removal of these agricultural chemicals. The present work aims to determine adsorption isotherms and rates of adsorption for five agricultural chemicals (CAT, TPN, isoprothiolane, 2,4-D and diazinon), humic substances, and the coexistence of agricultural chemicals and humic substances on activated carbon. The isotherms are correlated with the Freundlich equation in the concentration ranges investigated. Surface diffusion was considered to be dominant, and the diffusion coefficients increased with increasing amount adsorbed. The amount of agricultural chemicals adsorbed and the effective surface diffusion coefficients for the coexistence of agricultural chemicals and humic substances were lower than those for single agricultural chemicals.