As a comparison to airborne infrared (IR) flux measurements, ground-based sampling of fumarole and soil gases was used to characterize the quiescent degassing of CO₂ from Oldoinyo Lengai volcano. Aerial and ground-based measurements are in good agreement: ∼75% of the aerially measured CO₂ flux at Lengai (0.05–0.06 × 10¹² mol yr⁻¹ or 6000–7200 tonnes CO₂ d⁻¹) can be attributed to seven large crater vents. In contrast to Etna and Vulcano Island, where 15–50% of the total CO₂ flux emanates diffusely through the volcanic flanks, diffuse emissions were measured only within 500 m of the crater rim at Lengai, contributing <2% of the total flux. The lack of extensive flank emissions may reflect the dimensions of the magma chamber and/or the lack of a shallow fluid flow system. Thermodynamic restoration of fumarole analyses shows that gases are the most CO₂-rich and H₂O-poor reported for any volcano, containing 64–74% CO₂, 24–34% H₂O, 0.88–1.0% H₂, 0.1–0.4% CO and <0.1% H₂S, HCl, HF, and CH₄. Volatile emissions of S, Cl, and F at Oldoiyno Lengai are estimated as 4.5, 1.5, and 1.0 × 10⁷ mol yr⁻¹, respectively. Accuracy of the airborne technique was also assessed by measuring the C emission rate from a coal-burning power plant. CO₂ fluxes were measured within ±10% near the plant; however, poor resolution at increased distances caused an underestimation of the flux by a factor of 2. The relatively large CO₂ fluxes measured for alkaline volcanoes such as Oldoinyo Lengai or Etna may indicate that midplate volcanoes represent a large, yet relatively unknown, natural source of CO₂.