Abstract Although tert-butyl alcohol (TBA) has not been used as a fuel oxygenate in Orange County, California, the concentrations of TBA in ground water at gasoline spill sites are high compared those of the conventional fuel oxygenate methyl tert-butyl ether (MTBE). In the year 2002, the geometric mean concentration of TBA was 839 g/L compared to 627 g/L for MTBE. There are two plausible sources of TBA, the TBA originally present in the gasoline and biodegradation of MTBE to TBA. Thirteen sites where the concentration of TBA was >10 mg/L were selected for study. Biodegradation was evaluated by determining the stable carbon isotope ratio (13C) of MTBE in the ground water. If MTBE was biodegraded, the residual MTBE should be enriched in the heavier carbon isotope 13C. The historical highest concentration of MTBE in each monitoring well and the 13C of MTBE in the ground water were used to estimate the concentration of TBA produced from biodegradation of MTBE. Then, the estimated concentration of TBA was compared to the measured concentration. At each site, comparisons were made for the well in the source area, and the well in the plume outside the source area, that had the highest concentration of TBA. In the wells in the source areas, the estimate of TBA production accounted for the majority of TBA at 4 of the 13 sites. In the wells in the plume outside of the source area, the estimate accounted for the majority of TBA at six sites.