Equations are developed describing the rate of change of carbon isotopic ratios in the atmosphere and oceans in terms of δ13C quantities. The equations enable one to perform calculations directly with δ and ϵ quantities commonly reported in the literature. The main cause of the change occurring today is the combustion of fossil fuel carbon with lower δ13C values. The course of this isotopic anomaly in atmosphere and oceans can provide new constraints on the carbon budgets of these reservoirs. Recently published δ13C isotopic data of total inorganic carbon in the oceans [Quay et al., 1992] appear to lead to incompatible results with respect to the uptake of fossil fuel CO2 by the oceans if two different approaches to the data are taken. Consideration of the air-sea isotopic disequilibrium leads to an uptake estimate of only a few tenths of a gigaton C (Gt, for 1015 g) per year, whereas the apparent change in the ocean δ13C inventory leads to an estimate of more than 2 Gt C yr−1. Both results are very uncertain with presently available data. The isotopic ratio has the advantage that the signal-to-noise ratio for the measurement of the uptake of the isotopic signal by the oceans is better than for the uptake of total carbon, The drawback is that isotopic exchange with carbon reservoirs that are difficult to characterize introduces uncertainty into the isotopic budget. The accuracy requirements for the measurements are high, demanding careful standardization at all stages.