The article presents the results of a study of peroxidase oxidation of quercetin in the xylem of 40‑year-old birch trees of 2 forms differing in wood texture – silver birch (Betula pendula Roth var. pendula) and Karelian birch (B. pendula var. carelica (Mercl.) Hamet-Ahti), in the pH range of 4 to 10. Quercetin was used as a known model substrate for the study of peroxidase processes. The quercetin absorption maxima for the studied pH range were determined. The specific features of the reaction in acidic, neutral and alkaline media were considered for the studied objects. The time dependence of peroxidase activity is presented, as well as the pattern of peroxidase activity change in the pH range. Quercetin oxidation by peroxidase in the xylem of silver birch and Karelian birch was quantified. Acidic isoforms of the enzyme were shown to have higher activity compared to basic ones in both birch forms. In acidic conditions, peroxidase activity did not differ between silver and Karelian birch. In basic conditions, peroxidase activity was significantly higher in Karelian birch xylem. The resultant data generally indicate a higher oxidative capacity of peroxidase in Karelian birch xylem owing to a more active participation of all isoforms in the oxidation of quercetin. It is hypothesized that plant tissues with nonstandard enzymatic activity may be sources of biologically active substances and be used in biocatalysis processes.