We have detected circumstellar absorption lines of the 12CN and 13CN violet and red system in the spectrum of the post-AGB star HD 56126. From a synthetic spectrum analysis, we derive a Doppler broadening parameter of b = 0.51 ± 0.04 km s-1, 12CN/13CN = 38 ± 2, and a lower limit of 2000 on 12CN/14CN and 12C14N/12C15N. A simple chemical model has been computed of the circumstellar shell surrounding HD 56126 that takes into account the gas-phase ion-molecule reaction between CN and C+. From this we infer that this reaction leads to isotopic fractionation of CN. Taking into account the isotopic exchange reaction and the observed 12CN/13CN, we find 12C/13C ∼ 67 (for Tkin = 25 K). Our analysis suggests that 12CN has a somewhat higher rotational temperature than 13CN: Trot = 11.5 ± 0.6 and 8.0 ± 0.6 K, respectively. We identify possible causes for this difference in excitation temperature, among which is the N″ dependence of the isotopic exchange reaction. © 1998. The American Astronomical Society. All rights reserved.