When present, an epikarst represents the starting point (the first karst compartment) of water flow through a karst system. The air characteristics in a karst, and especially in an epikarst, determine the initial water characteristics, e.g., water aggressiveness, which depends on the partial pressure of CO 2 ( P CO 2 ) in equilibrium with water. This paper proposes a method to estimate P CO 2 in epikarst air using spring water measures as HCO 3 - , temperature, pH and δ 13 C of Total Dissolved Inorganic Carbon (TDIC) and cave air measures as P CO 2 and δ 13 C of CO 2 . This method accounts for the TDIC variations of δ 13 C that are caused by CO 2 degassing and calcite precipitation from water. The calculations are based on the influence of pH variations and carbon loss on δ 13 C of TDIC. Measurements are taken at two sites: Lascaux cave and the Cussac cave sites located in Perigord, southwest of France. Four water springs are presented in this case study: two springs from an epikarst compartment, one spring from an unsaturated zone and one spring from a saturated zone. The P CO 2 in epikarst air is estimated to be from 4.4% (44,000 ppm) in winter to 10% in summer. These values are higher than the values of air P CO 2 measured in the soil (0.27–1.60%) or in the caves (0.30–3.1%, up to 7.50% in some parts of Lascaux). We show that in epikarst air, P CO 2 and δ 13 C CO 2 are not constant values but vary annually with high P CO 2 and depleted values (−22.31‰ VPDB) in the winter and higher P CO 2 and more depleted values in the summer (−24.20‰ VPDB).