Gallic acid and hydroquinone have been identified as the major allelochemicals of the known allelopathic plant Polygonella myriophylla. Both of these compounds occur in the foliage as glycosides. Quercetin and rhamnetin were identified as the major flavonoid constituents, but in much lower concentration. The behavior of gallic acid, hydroquinone, the hydroquinone glycoside arbutin, and benzoquinone in sterile and nonsterile soil from beneath Polygonella was investigated. Sterilization effectively stabilized arbutin, hydroquinone, and gallic acid. Concentrations of benzoquinone rapidly diminished in sterilized soil, and the compound was almost completely gone after 7 days. In nonsterile soils, all four compounds degraded rapidly. The order of persistence was hydroquinone > benzoquinone > gallic acid > arbutin. Persistence was rate-dependent. Arbutin degraded to hydroquinone, and benzoquinone formed as a degradation product of hydroquinone. Hydroquinone was also observed as a degradation product of benzoquinone. Benzoquinone degrades rapidly by nonmicrobial oxidative processes. These results support the hypothesis that microbial and nonmicrobial oxidative transformations of soil allelochemicals are crucial in mediating the allelopathic effects of Polygonella myriophylla.