The atmospheric fluxes of methane (CH4), carbon dioxide (CO2), and reactive organic gases (ROGs) were determined for natural terrestrial petroleum seeps in the Upper Ojai Valley, California, by measuring the emission rates from five vents and scaling these measurements with the known distribution of seeps within the valley. The Upper Ojai Valley seeps emit about 55 m3/day (1942 ft3/day) of gas, of which about 15 m3/day (529 ft3/day; 3.6 Mg/yr) is CH4, about 40 m3/day (1412 ft3/day; 27 Mg/yr) is CO2, and less than 0.05 m3/day (1.765 ft3/day; 0.04 Mg/yr) are ROGs. CH4 and ROG fluxes in the Upper Ojai Valley are, respectively, three and five orders of magnitude less than at the well-characterized Coal Oil Point field, a large offshore seep field located approximately 70 km (43 mi) to the west of the valley. The CO2 flux from these two fields is about the same. The compositions and δ13C values of seep and reservoir gases were also quantified and indicate extensive biodegradation of gaseous hydrocarbons and the input of isotopically enriched CO2 during ascent from the reservoir. Unlike the nearby CH4-dominated marine seeps, the largest percentage of gas emitted by seeps in the Upper Ojai Valley is CO2.