Abstract Quinclorac and 13 synthesized esters were monitored for toxicity in foliar- and soil-treated leafy spurge plants. Foliar treatment at 0.5 μmol/plant with quinclorac resulted with 100% mortality, whereas treatment with esters at 2 μmol/plant showed initial toxicity symptoms, but the plants recovered with time (24 weeks). Foliar toxicity symptoms and mortality increased throughout 24 weeks after soil treatment at 0.4 μmol/ plant with the higher MW esters (C5-C16); however, toxicity symptoms decreased and plants recovered after quinclorac or low MW ester treatments. [ 14C]Quinclorac esters also were monitored to ascertain if toxicity data could be related to ester mobility and/or metabolism. Foliar application showed volatile losses with the low MW esters; complete metabolism of esters occurred inside the treated leaves by 7 days, but little or no quinclorac was observed as a result; and little radiolabel migrated from the leaves. Soil application showed that the esters were metabolized by soil microorganisms through a series of ω- and β-oxidations and not by simple ester hydrolysis. These intermediate metabolites were identified by MS and NMR analyses. The C6 and C8 esters were metabolized to the quinclorac ester of 4-hydroxybutryic acid as a major intermediate in soil before conversion to quinclorac. The rate of quinclorac release from the various esters was dependent on the number of odd vs even carbons in the ester side chain; the C7, C5, and C8 esters were metabolized to quinclorac at relative rates of 16, 3.2, and 1, respectively. Quinclorac acid was leached readily from soil with water, but the C8 ester was adsorbed. As the higher MW esters released quinclorac in soil over time, concomitantly, the toxicity of leafy spurge after treatment with these esters also increased and was superior to quinclorac after 24 weeks. The slow release of quinclorac from the ω/β-oxidations of these esters appeared to increase efficacy in soil-treated leafy spurge.