Deuterium-protium fractionation factors were determined for benzylamine and benzylammonium ion by the traditional H-1 NMR method and for the benzylammonium ion by a newly devised C-13 NMR method. The results, phi PhCH(2)NL(2) = 0.958 +/- 0.070 and phi PhCH(2)NL(3)+ = 1.081 +/- 0.019, when combined with the solvent isotope effect on the ionization of benzylammonium ion, also determined here, K-H/K-D = 3.36 +/- 0.13, give Phi(PhCH2NL3)+ = 0.80 +/- 0 13 as the medium effect for transfer of this ion from H2O to D2O. These results show that introduction of a positive charge into an N-L bond does not decrease its fractionation factor as is the case for O-L bonds, where phi(OL)+/phi(OL) = 0.7. This difference is attributed to the tetrahedral structure of ammonium ions and the consequent stiffness of their bond-bending vibrations.