Seedlings of closely related birch species Betula pendula and B. resinifera, which have moderate similarity in their phytochemical composition, were exposed to a SO2 concentration of 200 μg m−3 for 24 days to detect their sensitivity and possible induced changes in the amount of primary and secondary phytochemicals. The species deviated from each other in the production of biomass and phytochemicals. The SO2 pollution induced no changes in growth nor in the soluble sugar concentrations in the species. However, the chromatographic analyses showed the moderate reduction in the amount of antioxidative and defensive phenolics, i.e. myricitrin, (+)-catechin, 3,4'-dihydroxypropiophenone and proanthocyanidins in both birch species. These results indicate more pronounced sensitivity of secondary metabolism to SO2 pollution compared with primary metabolism, and the reduced tolerance of birches to other oxidative air pollutants as well as to herbivores in polluted environments.