The alternation of ice and warm ages is connected to glacial/interglacial CO2 concentration changes of approximately 80-100 ppmv with significant fine structure during Termination I [Monnin et al., 2001]. Changes in the carbon isotopic signature of CO2 during that time are expected to add to our understanding what processes were responsible for the observed CO2 changes. First measurements revealed a glacial/interglacial change in 13CO2 of 0.2-0.3 / [Leuenberger et al., 1992] but significantly higher variations during the termination [Smith et al., 1999]. Using the so called Keeling plot approach ( 13C = a/CO2 + b, where b is taken as representative of the isotopic signature of carbon added or extracted from the atmosphere) it was concluded that the terrestrial biosphere was of major importance for CO2 changes in the glacial and the Holocene [Smith et al., 1999]. However, this approach known from terrestrial carbon cycle research represents essentially a carbon isotopic mass balance of a two reservoir system and its application on paleoclimatic CO2 changes is not straightforward. Here we revisit the Keeling plot approach on paleoclimatic time scales using ice core observations, theoretical considerations and modeling results. Based on output of transient model runs from our global carbon cycle model BICYCLE during the last transition [Köhler et al., 2005] we constrain the conclusions to be drawn from ice core 13CO2 data and Keeling plot analyses [Köhler et al., 2006].