Measurements of the CO2 concentration of air trapped in Greenland Ice Sheet Project Two ice show an increase from approximately 190 ppm to 270 ppm during the 8000 years between the end of the last glacial maximum and the beginning of the Holocene. This increase is in agreement with other parallel records from both Greenland and Antarctica, although the GISP2 data exhibit considerable variability over periods as small as 50-100 years and show that CO2 can be affected by processes occurring within the ice after bubble formation. The in-situ reactions which contribute to the modification of the concentrations of paleoatmospheric CO2 may include both CO2 uptake as well as CO2 production. The existence of two major episodes of CO2 enrichment, in addition to the variability of the data, prevents a precise determination of the progression of the increase in atmospheric CO2, although the 80 ppm rise between 18.0 kyBP and 10.4 kyBP is evident. These results strongly suggest that some intervals of the GISP2 CO2 record must be interpreted in the context of the chemical composition of the ice, and that records of atmospheric CO2 in polar ice can be considered accurate only if the ice is either (essentially) carbonate-free or contains abundant carbonate.