Observations of the 1→0 transition of the 12C and 13C isotopomers of CO and CN at 1.2 and 3 mm have been conducted towards various circumstellar envelopes, using the Kitt Peak 12m antenna and the Submillimeter Telescope (SMT) of the Arizona Radio Observatory (ARO). These observations have been used to establish 12C/13C ratios in these objects. CN is particularly useful in this regard because its spectra exhibit hyperfine structure from which accurate opacities can be evaluated. Observations include a variety of hydrogen-deficient stars, supergiants, Li-rich, M, S, and C stars. Measurements towards Asymptotic Giant Branch (AGB) stars (e.g. IRC+10216, CRL618, CRL2688, and IRC+40540), suggest preliminary 12C/13C isotope ratios in the range of 20-76, while in supergiants, 12C/13C ∼ 3-10. Theory predicts that carbon-12 is primarily formed in massive stars that evolve into Type II supernovae, while carbon-13 is produced from the CNO cycle occurring in intermediate mass AGB stars. Ratios determined for these objects with multiple molecular species help constrain nucleosynthesis models and provide insight into the 12C/13C Galactic gradient measured in molecular clouds, as well as local variations of this ratio found in the interstellar medium.