The CO 2 concentration of the air in Postojna Cave (400–7900 ppm) is found to be induced by CO 2 sources (human respiration contributing ~ 20,000–58,000 ppm per breath, outgassing of dripwater and water seeping from the vadose zone/epikarst with a p CO 2 values of 5000–29,000 ppm, and underground Pivka River having p CO 2 at 2344–4266 ppm) and CO 2 dilution (inflow of outside air with a CO 2 concentration of ~ 400 ppm). Measurements show that sinking Pivka River has the lowest CO 2 concentration among plausible CO 2 sources but still continuously exceeds the surrounding cave air CO 2 concentration. During the winter months, intensive ventilation reduces the cave air CO 2 concentration to outside levels (~ 400 ppm), even in the centre of the cave system. CO 2 dilution is less pronounced in summer (CO 2(min)  ≈ 800 ppm), since the ventilation rate is not as strong as in winter and the outside air that enters the cave through breathing holes and fractures is enriched with soil CO 2 . During spring and autumn, the daily alternation of the ventilation regime with a smaller rate of air exchange results in yearly cave air CO 2 peaks of up to ~ 2400 ppm. Some dead-end passages can be much less affected by ventilation, resulting in a cave air CO 2 concentration of up to 7900 ppm. The strongest diurnal CO 2 peaks due to human respiration were recorded during the spring holidays (increase of up to 1300 ppm day −1 ), compared to considerably smaller summer peaks despite peak visits (increase of ~ 600 ppm day −1 ).