The effect of different initial in-package O2 and CO2 concentrations (2.5 kPa of O2 + 5 kPa of CO2, 10 kPa of O2 + 5 kPa of CO2, and 21, 60, and 80 kPa of O2) on the antioxidant potential of fresh-cut tomatoes was investigated. Changes in individual phenolic compounds, individual carotenoids, vitamin C, and antioxidant capacity as well as in O2, CO2, and ethylene headspace concentrations inside packages were assessed for 21 days at 4 degrees C. High-oxygen and passive atmospheres induced higher production of carotenoids and phenolic compounds. The degradation of the initial content of vitamin C was highly promoted by the presence of oxygen. Lower hydrophilic antioxidant capacity (DPPH assay) was obtained in tomato slices stored under 80 kPa of O2, whereas the antioxidant capacity of the lipophilic fraction was enhanced with oxygen availability inside headspace packages. Therefore, 2.5 kPa of O2 + 5 kPa of CO2 atmospheres not only reduced the formation of carotenoids but also maintained vitamin C in fresh-cut tomatoes.