In laboratory microcosms using salt marsh soils and in field trials, it was possible to monitor and quantify crude oil mineralization by measuring changes in CO2 δ13C signatures and the rate of CO2 production. These values are easy to obtain and can be combined with simple isotope mass balance equations to determine the rate of mineralization from both the crude oil and indigenous carbon pool. Hydrocarbon degradation was confirmed by simultaneous decreases in alkane-, isoprenoid-, and PAH-hopane ratios. Additionally, the pseudo-first-order rate constants of alkane degradation (0.087 day-1) and CO2 production (0.082 day-1) from oil predicted by the δ13C signatures were statistically indistinguishable. The addition of inorganic nitrogen and phosphate increased the rate of mineralization of crude oil in aerated microcosms but had no clear effect on in situ studies. This procedure appears to offer a means of definitively quantifying crude oil mineralization in a sensitive, inexpensive, and simple manner in environments with appropriate background δ13C signatures.