Two isotopic chemical reactions, Ne* + NH_3, and Ne* + ND_3, have been studied at low collision energies by means of a merged beams technique. Partial cross sections have been recorded for the two reactive channels, namely, Ne* + NH_3 → Ne + NH^+_3 + e −, and Ne* + NH_3 → Ne + NH^+_2 + H + e −, by detecting the NH^+_3 and NH^+_2 product ions, respectively. The cross sections for both reactions were found to increase with decreasing collision energy, E coll , in the range 8 μeV < E coll < 20 meV. The measured rate constant exhibits a curvature in a log(k)-log(E coll ) plot from which it is concluded that the Langevin capture model does not properly describe the Ne* + NH_3 reaction in the entire range of collision energies covered here. Calculations based on multichannel quantum defect theory were performed to reproduce and interpret the experimental results. Good agreement was obtained by including long range van der Waals interactions combined with a 6-12 Lennard-Jones potential. The branching ratio between the two reactive channels, Γ=[NH^+_2]/([NH^+_2]+[NH^+_3]), is relatively constant, Γ ≈ 0.3, in the entire collision energy range studied here. Possible reasons for this observation are discussed and rationalized in terms of relative time scales of the reactant approach and the molecular rotation. Isotopic differences between the Ne* + NH_3 and Ne* + ND_3 reactions are small, as suggested by nearly equal branching ratios and cross sections for the two reactions.