The molecular complexity of imazaquin and presence of ionizable functional groups limits the ability to predict sorption behavior from single soil parameters such as organic carbon content. Partition coefficients (K p ) for both neutral and anionic forms of imazaquin as well as the effects of solution ionic strength and composition were investigated to more adequately describe sorption of imazaquin in soil. Soils representing a range of characteristics were evaluated, including soils with permanent negative or variable surface charge. Imazaquin retention resulted from combined sorption for the neutral (K oc,n , 1,110 ฯฎ 80 L/kg) and anionic (K oc,a , 38 ฯฎ 20 L/kg) forms. Imazaquin sorption was best correlated to soil organic carbon content and soil-solution pH. However, results indicated that positively charged Fe 2ฯฉ and Al 3ฯฉ oxyhydroxides contribute to sorption of the organic anion; thus mineral surfaces contributed to sorption in soils with low organic carbon content. The effects of electrolyte matrices on imazaquin sorption were accounted for by concomitant changes in pH. However, enhanced imazaquin sorption was observed with increasing ionic strength for soils where pH-induced changes in speciation were negligible, indicating the role of mechanisms other than weak hydrophobic interactions. Addition of H 2 P significantly decreased imazaquin sorption, especially in weathered soils.