A model of carbon and oxygen isotope fractionation is described and model results presented. The talk will discuss the construction of the fractionation chemistry, and its application to observations of carbon-13 isotopologues in the interstellar medium. The method of deriving the main carbon isotopologue abundance from observations of the rarer carbon-13 species will be discussed in relation to the model results, as will the connection between the underlying 12/13 C ratio and the ratio observed in molecules. In both cases, it is found that current assumptions are valid for some species, but are invalid for others.