The content and nature of soil organic matter, and especially of its humic acid (HA) fraction, is ascertained to be a major factor for pesticide adsorption in soil. Organic amendment of soil is known to affect the amount and chemical properties of native soil HAs. This effect is thus expected to modify the quantitative and mechanistic aspects of pesticide adsorption to soil HAs. The binding mechanisms and adsorption isotherms of the pesticides alachlor [2-chloro-2′,6′-diethyl-N-(methoxymethyl)acetanilide], imazethapyr [(RS)-5-ethyl2-(4-isopropyl-4-methyl-5-oxo-2-imidazolin-2-yl)nicotinic acid], rimsulfuron [N-[(4,6-dimethoxypyrimidine-2-yl)carbamoyl]-3-(ethylsulfonyl)-2-pyridine sulfonamide], triallate (S-2,3,3-trichloroallyl diisopropylthiocarbamate), and chlordane (1,2,4,5,6,7,8,8-octachlor-2,3,3a,4,7,7a,-hexahydro-4,7-methano-indene) to HAs from two sewage sludges and a pig slurry and from a number of nonamended and amended soils have been studied in two distinct experiments. Binding mechanisms were investigated on HA pesticide interactions by use of chemical and spectroscopic methods including Fourier-transform infrared (FTIR), fluorescence and electron spin resonance (ESR). Adsorption isotherms of pesticides to HAs were obtained by using a batch equilibrium method coupled with high-pressure liquid or gas chromatographic analyses. Spectroscopic data suggest that moderately water-soluble, polar pesticides such as alachlor, imazethapyr and rimsulfuron are mainly adsorbed to HAs by multiple binding mechanisms, including H-bonding and ionic and charge-transfer processes, whereas the low water-soluble, nonpolar triallate and chlordane preferentially bind to Has by hydrophobic bonding. The highly aliphatic, low humified HAs from organic amendments tend to bind pesticides by hydrophobic bonding, whereas the well humified soil HAs would prefer chemical binding forms. In general, alachlor and triallate exhibit linear isotherms, imazethapyr nonlinear Freundlich isotherms of the C-type, and rimsulfuron and chlordane Langmuir-type isotherms in their adsorption to HAs. The extent of adsorption of the various pesticides onto HAs follows the order: chlordane〉 triallate 》 rimsulfuron 》 imazethapyr 〉 alachlor. The adsorption capacity of the various HAs for the different pesticides is variable and related to the composition and structural and functional properties of the HA. In conclusion, organic amendment of soil has been shown to affect the magnitude and modify the mechanisms of pesticide adsorption to soil HAs.