The adsorption of metolachlor, acetochlor, pretilachlor and butachlor, as a group of acetanilide herbicides, on eight soils with various physical and chemical properties was studied. The adsorption isotherms ยฎt the Freundlich equation well. The extent of adsorption increased in the order: metolachlor acetochlor pretilachlor `butachlor. The product of the Freundlich adsorption constants, K f (1/n), showed good correlation with organic matter content (OM) of soils for each of these herbicides, suggesting that the latter is the main factor controlling the adsorption process of these acetanilide herbicides. Multivariant correlation regression between K f (1/n) and two factors, water solubility (S w ) of herbicides and OM, was also performed. K f (1/n) correlated well with 1/S w and OM/S w , showing that high S w corresponds to a weak tendency to adsorb on soils. IR spectra and ESR parameters conยฎrmed that multifunctional H bonds and charge-transfer bonds between humic acids (HA) and the herbicides were the main adsorption mechanisms of the latter. The ability of herbicides to form these adsorption bonds with HA increased in the same order as the extent of adsorption.