This study reports the influence of a 20th century pollution signal recorded in the δ¹³C and δ¹⁸O of absolutely dated tree rings from Quercus robur and Pinus sylvestris from southern England. We identify a correspondence between the inter-relationship and climate sensitivity of stable isotope series that appears to be linked to recent trends in local SO₂ emissions. This effect is most clearly exhibited in the broadleaved trees studied but is also observed in the δ¹³C values of the (less polluted) pine site at Windsor. The SO₂ induced stomatal closure leads to a maximum increase of 2.5‰ the isotope values (δ¹³C). The combined physiological response to high pollution levels is less in δ¹⁸O than δ¹³C. The SO₂ signal also seems to be present as a period of reduced growth in the two ring-width chronologies. Direct, quantitative correction for the SO₂ effect represents a significant challenge owing to the nature of the records and likely local plant response to environmental pollution. Whilst it appears that this signal is both limited to the late industrial period and demonstrates a recovery in line with improvements in air quality, the role of atmospheric pollution during the calibration period should not be underestimated and adequate consideration needs to be taken when calibrating biological environmental proxies in order to avoid development of biased reconstructions.