[en] Laboratory measurements presented here using the ion cyclotron resonance technique yield a rate constant 2 x 10-10 cm3s-1 at 300 K for the isotope exchange 13C++12CO→12C++13CO. According to the usual ideas about ion-molecule reactions, this rate constant should also be appropriate at temperatures approximately-less-than100 K. Then observed the 13C/12C ratio obtained from radio observation of interstellar molecules may be either larger or smaller than the actual value in the interstellar medium by factors of 2 or so. If the ratio is altered from the actual interstellar value, it will not be the same in all molecules and CO will tend to have the highest value. The chief astronomical uncertainty for the occurrence of this isotope fractionation is the abundance of ''unobservable'' molecules which can react rapidly with C+: e.g., O2, H2O, CO2, and CH4. If their abundance is greater than about one-tenth that of CO, the isotope fractionation will be inhibited