International audience ; Although natural CO 2 -rich groundwaters of eastern Belgium have been known for centuries, the exact origin of their gas is still unclear. This paper presents the results of a sampling campaign in Belgium (Spa, Stoumont, Malmedy): 30 samples of both carbogaseous and non-carbogaseous groundwaters were analyzed for major elements, CO 2 content and carbon isotopic composition. Among them, 13 samples were also analyzed for 3 He /4 He and 4 He /20 Ne ratios. The combination of δ 13 C (between ca. −9 ‰ VPDB 1 and −2 ‰ VPDB), CO 2 /3 He ratio (between 1.9×10 8 and 2.9×10 9 ) and 3 He /4 He (between 0.92 and 2.70 Ra) shows with a high level of confidence that the CO 2 in the carbogaseous groundwater of Spa and Bru has a mantle origin. It can likely be attributed to the degassing of mantle from the neighboring Eifel volcanic fields, located at a distance of 100 km eastwards. The identity and nature of the deep-rooted fractures that act as CO 2 transport pathways to the surface are still to be clarified, but several major thrust faults exist in the Rhenish Massif and could connect the Eifel volcanic fields with the studied area.