Polymeric dyes are aromatic hydrocarbon compounds that are similar to aromatic soil pollutants and inhibit plant growth. Polymeric dye-tolerant plants may be useful in phytoremediation because they could provide a rhizosphere suitable for colonization by microbes that are efficient degraders of aromatic compounds. Plants that produce high levels of phenolic compounds may be more tolerant of aromatic contaminants because of their inherent mechanisms for degrading or sequestering endogenous aromatic compounds. A clonal line of oregano (Origanum Vulgare) O-1 that is highly tolerant of the polymeric dye R-478 has been isolated. Over a time course of exposure to 0.1% Poly R-478, O-1 plants showed a progressive decrease in total phenolic compounds, whereas peroxidase activity and free proline levels increased. The inverse relationship between phenolics and peroxidase activity suggests a role for peroxidase-mediated lignification in polymeric dye tolerance. The increased level of free proline in response to polymeric dye exposure suggests that phenolic synthesis requires proline synthesis and/or oxidation. These results support our working model that aromatic pollutant-tolerance may be regulated through the proline-linked pentose phosphate pathway, a primary metabolic pathway.