The isotopic composition of carbonates has been widely used as a proxy for reconstructing Earth’s past environmental conditions, which requires the preservation of the isotope composition of the primary precipitate or knowledge about its transformation pathway. In this context, the formation of crystalline carbonates via amorphous precursors has been identified as a common formation process for natural carbonates, where the impact to the isotopic composition of the final CaCO3products is insufficiently known to date. This study focusses on the stable carbon isotope fractionation during the transformation of amorphous calcium carbonate (ACC) into crystalline carbonate in the presence/absence of atmospheric CO2induced by (i) humidity (from 33 to 95 % RH) at a temperature of 5 and 20 °C and (ii) solid-state transformation of ACC to calcite induced by heating up to 500 °C. During the crystallization of ACC at very low water ratios, induced by humidity, and in the presence of CO2, δ13C values of the solids increased with reaction time, where the crystalline carbonate-ACC enrichment factors (13εcc−ACC) range between 2.0 and 8.4 ‰ at the final stage. This13εcc−ACCevolution reflects the incorporation of ambient CO2(g) alongside carbonate ions released during ACC dissolution. In contrast, without CO2(g) the crystalline CaCO3almost matches the isotope composition of the ACC (13εcc−ACC= −0.4 ± 0.3 ‰), thus indicating non-significant CO2degassing and/or subsequent re-equilibration trend. The solid-state transformation of ACC to calcite during heat-induced dehydration yields a slightly negative enrichment factor (13εcalcite−ACC= −0.8 ± 0.3 ‰) that may reflect carbon isotope fractionation due to rapid recrystallization-rate kinetics. Our results suggest that the carbon isotope compositions of carbonates formed via an amorphous precursor at low water/solid ratios (e.g., in caves, soils or biominerals) may not accurately reflect the original conditions of the carbonate formation, hence paleoenvironmental conditions should be interpreted with care.