Measurements of meltwater pH from annual layers of South Pole firn and ice samples ranging in age from 40 to 2000 years B.P. show that precipitation at this remote site has a higher natural acidity than that expected from atomospheric equilibrium with CO2. The average pH of deaerated (CO2-free) samples was 5.64¿0.08, while air-equilibrated samples averaged 5.37¿0.08, a pH that is about a factor of two more acidic than the expected background pH of 5.65. The observed ''excess'' acidity can be accounted for by SO2-4 and NO-3 levels in the samples originating from non-anthropogenic H2SO4 and HNO3. Because of the presence of these naturally occurring acids in South Pole precipitation, a pH of 5.4 is considered a more representative baseline reference pH for acid precipitation studies. ¿ American Geophysical Union 1987