Carbonates formed from hyperalkaline aqueous solutions at the Earth́s surface are known to bear the most extreme disequilibrium isotope signatures reported so far in nature. We present here the results for stable carbon (C), oxygen (O), and barium (Ba) isotope fractionation during the precipitation of witherite (BaCO3) induced by the chemical absorption of atmospheric carbon dioxide (CO2) into an aqueous hyper-alkaline solution (at 4° and 21 °C; 1 atm total pressure). Independent from temperature, the barium carbonate formation was associated with a substantial enrichment of the lighter C and O isotopes in the solid compared to the atmosphere (C, O), close to previous results found in experiments and nature. A new approach is introduced to explain oxygen isotope fractionation upon hydroxylation of CO2. With Ba isotope enrichment factors between −0.45 and −0.53‰ (138/134ε) or −0.34 and −0.40‰ (137/134ε), respectively, the synthesized BaCO3 displays the highest kinetic enrichment of the light Ba isotope in the carbonate solid reported so far.