Four humic substances were extracted from a peat (HA1) and a volcanic soil (HA2), an oxidized coal (HA3), and a lignite (HA4). The four humic materials presented distinct differences in their chemical and physical−chemical characteristics as assessed by chemical methods, 13C-NMR spectroscopy, and high-performance size exclusion chromatography. The interactions between these well-characterized and purified humic substances and the widely used glyphosate [N-(phosphonomethyl)glycine] herbicide were studied by means of adsorption isotherms. Adsorption of glyphosate was found to be surprisingly high and followed the order HA1 > HA2 > HA3 ≥ HA4. Humic extracts from soil adsorbed glyphosate even more than clay minerals, thereby indicating that the interactions with humic substances, in either a solid or dissolved form, are far more important than previously believed. Adsorption is explained by the multiple hydrogen bondings which can occur among the various acidic and oxygen-containing groups of both molecules. ...