Water-soluble coal-based acids (WSCA) obtained by oxidation of Wuchuan weathered lignite (W-WSCA) or by extraction from the Jincheng weathered coal (J-WSCA), enhanced the biological activity of tribenuron-methyl, an herbicide. in the presence of the WSCA, the ED90 values for tribenuron-methyl were increased by a factor of 4.28 (W-WSCA) or 2.52 (J-WSCA). FTIR analysis indicated that the main interaction between WSCA and herbicide was via hydrogen bonds. NMR proved the results of FTIR. The established molecular dynamics simulation indicated that the more open structure favored the interaction between the herbicide and additive. The strong interaction between herbicide and additive and consequent marked change in herbicide structure are probably related to the improved biological activity of the herbicide. (c) 2007 Elsevier B.V. All rights reserved.