Abstract: This data set includes d15N, d18O/2, dO2/N2/4, d40Ar/4, d38/Ar/2, d84Kr/48, and d132Xe/96 values for air drawn from the top 15 to 50 m of firn at the South Pole (summer and winter 1998) and a site at Siple Dome (summers 1996 and 1998). Data also include related firn temperature measurements. The objective of this research was to better understand thermal fractionation processes affecting records of atmospheric history from firn and ice core gases. Recent work (e.g., Severinghaus and Brook, 1999) has exploited trapped air in ice and deep firn as a record of past atmospheric composition and climate change. Interpretation of these paleoclimate archives is complicated by artifacts of thermal diffusion, a process in which heavier gases migrate down temperature gradients toward colder regions in the firn. Seasonal temperature change at the snow surface creates strong temperature gradients in the top few meters of the firn, which cause isotopic fractionation of firn gases. A specific goal of this research is to identify any long-term effects of seasonal temperature fluctuations on firn air isotopic anomalies.