We quantify the gas pressure and concentration of a gas-charged acidic pit lake in SW Spain. We measured total dissolved gas pressure, carbon dioxide (CO₂) concentration, major ion concentration, isotopic composition of dissolved inorganic carbon (δ¹³C_DIC), and other physicochemical parameters. CO₂ is the dominant dissolved gas in this lake and results mainly from carbonate dissolution during the interaction of acidic water with wall rocks, followed by diffusive and advective transport through the water column. The δ¹³C_DIC values suggest that the biological contribution is comparatively small. Maximum CO₂ concentrations higher than 0.1 M (5000 mg/L) have been measured, which are only comparable to those found in volcanic crater lakes. The corresponding gas pressures of CO₂ alone (pCO₂ 3.6 bar) imply 60% saturation relative to local pressure at 50 m depth. High CO₂ concentrations have been observed in other pit lakes of the region. We recommend gas-specific monitoring in acidic pit lakes and, if necessary, the design of feasible degassing strategies.