Proton exchange between [N-15]ammonium ion and ammonia was studied by quantitative two-dimensional NMR exchange spectroscopy (2D-EXSY) in H2O-D2O. In this mixture the ammonium ion exists as the five isotopologues (NH4+)-N-15, (NH3D+)-N-15, (NH2D2+)-N-15, (NHD3+)-N-15, and (ND4+)-N-15, four of which are distinguishable by H-1 NMR with deuterium decoupling. Ammonia-catalyzed exchange is observed even near pH 1, and exchange among all sites is detected as 2D-EXSY cross peaks connecting each pair of species. Exchange with water also occurs and its rate was measured by saturation transfer. Analysis of peak intensities led to the matrix R, containing all site-to-site rate constants k(nm). Averaging 44 of these gave (1.6 +/- 0.2) X 10(8) M(-1) s(-1) as the second-order rate constant per proton for NH3-catalyzed exchange. The six pairs of rate constants that lie adjacent to the diagonal of R contain kinetic information regarding the primary deuterium kinetic isotope effect. From 12 separate estimates a value of k(H)/k(D) = 1.8 +/- 0.2 was calculated for this direct nitrogen-to-nitrogen proton transfer. The low kinetic isotope effect is discussed in terms of a reaction that is nearly encounter-controlled.