The spread of Russian (Acroptilon repens) and spotted knapweed (Centaurea maculosa), two invasive weed species, is wreaking havoc upon native plant ecosystems in the western US at tremendous environmental and economic costs. Their successful spread has been attributed to the exudation of allelopathic chemicals by their roots into surrounding soil that are phytotoxic to native plant species. These toxic allelochemicals have recently been isolated and identified as (-)-catechin and 7,8 benzoflavone, exuded by spotted and Russian knapweed, respectively. Their toxicity is a function of their bioavailability in the soil solution which is regulated by soil sorption (retention) processes. Very little is known about the sorption of these allelochemicals in soil. Specifically, under what soil conditions (soil constituents (i.e., organic matter, clays, and oxides) and pH) do these chemicals maximally sorb? The work in this proposal will seek to answer these questions. Understanding the conditions resulting in optimum sorption – or minimal bioavailability – of these allelochemicals could provide a means to arrest the spread of Russian and spotted knapweed.