In this study, we present the first experimental results for stable barium (Ba) isotope (137Ba/134Ba) fractionation during low-temperature formation of the anhydrous double carbonate BaMn[CO3]2. This investigation is part of an ongoing work on Ba fractionation in the natural barium cycle. Precipitation at a temperature of 21±1°C leads to an enrichment of the lighter Ba isotope described by an enrichment factor of−0.11±0.06‰ in the double carbonate than in an aqueous barium-manganese(II) chloride/sodium bicarbonate solution, which is within the range of previous reports for synthetic pure BaCO 3 (witherite) formation.