Elevated concentrations of soil CO2, ranging from 20% to 90%, from a magmatic source have been killing coniferous trees in the last decade in several distinct areas on the flanks of Mammoth Mountain, California. These areas of elevated soil CO2 provide a natural laboratory to examine how chemical weathering in soils responds to extremely high-CO2 concentrations in conjunction with possible changes in organic acids resulting from vegetation mortality. These volcanic ash soils have a particularly low resistance to chemical weathering due to the predominance of volcanic glass and other noncrystalline phases. Thus, decade-long exposure to anomalous conditions may result in differences in soil characteristics. Soil samples from within the high-CO2 region have been analyzed and compared to samples from outside the anomalous area in the adjacent healthy forest areas where background CO2 concentrations are less than 1%. Compared to the control soil, the high-CO2 soil has lower pH values (5.0 compared to 5.6), higher soil moisture content, and higher surface area. Dithionite-citrate and acid-oxalate extractants were less effective in leaching Al and Si from the high CO2 than from the control soil, indicating a distinct difference in the mineralogy of these soils. The observed differences between the high-CO2 and control soils are consistent with an enhancement of weathering intensity in the soil exposed to elevated CO2 concentrations. © 2002 Elsevier Science B.V. All rights reserved.