Uniformly /sup 14/C-ring-labeled prometryn (2-(methylthio)-4,6-bis(isopropylamino)-s-triazine) was incubated with an organic soil under laboratory conditions for 1 year. After exhaustive solvent extraction, the soil containing bound (nonextractable) /sup 14/C-labeled residues was fractionated into humic substances by alkali extraction followed by acid precipitation. A considerable proportion of /sup 14/C-labeled residues in humin and humic acid was comprised of the parent herbicide and its mono-N-dealkylated metabolite. However, the soluble fulvic acid fraction contained an appreciable amount of the 2-hydroxy analogue of prometryn. Thermoanalytical methods were used to obtain information on the nature and location of /sup 14/C-labeled bound residues in soil and humic materials. Exhaustive methylation of humin and humic acid released some of the bound /sup 14/C-labeled residues from these fractions. Bound /sup 14/C-labeled residues in aqueous suspensions of humin or humic acid were stable to UV light. However, UV irradiation of the soil suspension resulted in the release of some of the bound /sup 14/C-labeled residues which were subsequently decomposed to hydroxy analogues.