Climate variations and changes in sulphur and nitrogen deposition from the atmosphere influence the acid-base balance of sensitive lakes in a complex and site-specific way(1-3). For example, although lakes in several regions have shown a decline in sulphate concentration following reductions in atmospheric sulphate deposition(4-6), the expected recovery of pH and alkalinity has not always taken place, implicating an additional response to changes in the local climate. Here we report a study of 57 remote alpine lakes which shows that, between 1985 and 1995, lake pH and the concentration of sulphate, base cations and silica have increased, whereas inorganic nitrogen concentrations have decreased. This contrasts with atmospheric input trends, which have led to a decrease in sulphate and a slight increase in nitrogen deposition over the same period(7,8). We propose that the changes in lake chemistry are therefore likely to be caused by enhanced weathering and increased biological activity resulting from an increase in air temperature of about 1 degrees C since 1985. Our analysis of an alpine lake core covering a 200-year period provides further evidence for a strong positive correlation between pH and mean air temperatures, and thus for the high sensitivity of lakes at high altitudes and high latitudes to climate warming. In these remote locations, temperature effects, rather than acid deposition, appear to dominate changes in lake acidity.