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The mode of atrazine [6‐chloro‐N‐ethyl‐N′‐(1‐methylethyl)‐1,3,5‐triazine‐2,4‐diamine] adsorption to humic substances is still uncertain, mainly because of the large heterogeneity of humus and its poor molecular characterization. In this study, four humic fractions were isolated from a peat and a volcanic soil by three different extraction mechanisms: charge repulsion (NaOH extraction), ligand exchange (pyrophosphate extraction, Na4P2O7), and hydrogen‐bonding disruption (extractions by aqueous mixture of acetone and dymethyl sulphoxide, DMSO). These four humic fractions showed consistent differences in molecular structure between the two soils. Humic fractions extracted by acetone had the largest content of aliphatic C, whereas those obtained by the pyrophosphate solution were richest in aromatic C. Sorption isotherms of 14C‐labeled atrazine conducted on the four humic fractions showed that, for both soils, the order of adsorption was acetone > NaOH > DMSO > Na4P2O7, whereas the order for desorption was the reverse. The relation between results of atrazine interaction and the molecular structure of humic matter indicates that the aliphatic C content of soil organic matter may be one of the parameters controlling atrazine adsorption to soils. Hydrophobic interactions and conformational flexibility in the aliphatic portions of humic matter controlled the adsorption of atrazine in the interior of humic self‐associated aggregates and the degree of desorption found both in water and methanol. Conversely, the conformational rigidity conferred to humic fractions by a large content of aromatic moieties appeared conducive only to surface adsorption and thus to easier herbicide desorption.

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