© 2020 The Author(s). Published by Oxford University Press on behalf of Royal Astronomical Society.Viti, S.Fontani, F.Jiménez Serra, I.Unidad de Excelencia Científica María de Maeztu Centro de Astrobiología del Instituto Nacional de Técnica Aeroespacial y CSIC, MDM-2017-07372021-04-082021-04-082020-07-18Monthly Notices of the Royal Astronomical Society 497: 4333- 4345 (2020)0035-8711https://academic.oup.com/mnras/article-abstract/497/4/4333/5877251http://hdl.handle.net/20.500.12666/150In the interstellar medium carbon exists in the form of two stable isotopes 12C and 13C and their ratio is a good indicator of nucleosynthesis in galaxies. However, chemical fractionation can potentially significantly alter this ratio and in fact observations of carbon fractionation within the same galaxy has been found to vary from species to species. In this paper, we theoretically investigate the carbon fractionation for selected abundant carbon-bearing species in order to determine the conditions that may lead to a spread of the 12C/13C ratio in external galaxies. We find that carbon fractionation is sensitive to almost all the physical conditions we investigated, it strongly varies with time for all species but CO, and shows pronounced differences across species. Finally, we discuss our theoretical results in the context of the few observations of the 12C/13C  in both local and higher redshift galaxies.engAttribution-NonCommercial-NoDerivatives 4.0 Internationalhttps://creativecommons.org/licenses/by-nc-nd/4.0/ISM: abundancesISM: moleculesGalaxies: ISMMolecular TracersGasStar Formationinfo:eu-repo/semantics/article10.1093/mnras/staa22151365-2966info:eu-repo/semantics/openAccess