[en] Fractionation of carbon isotopes in interstellar clouds is treated in some detail. Acting together, the exchange reaction ¹³C⁺+¹²CO reversible ¹²C⁺+¹³CO+ΔE and the high vapor pressure of CO may cause a preferential loss of ¹²C from the gas due to freezing onto dust grains. The ¹³C/¹²C ratio for the carbon in the gas is then raised by factors approximately 2 to 4. Fractionation of deuterium is also discussed since the proposed process is similar to the carbon isotope exchange and here there is no question that fractionation is a major effect. Important information about physical conditions in dense clouds - the fractional electron density and CO abundance - can be derived from the fractionation process producing the enhanced DCO⁺/HCO⁺ ratio. Possible fractionation reactions for isotopes of other elements (oxygen, sulfur) are mentioned briefly. (Auth.)