The ion-molecule reaction NH3+ + ND3 has been studied at various collision energies (1 to 5 electron volts in the center of mass) with preparation of the NH3+ reagent in two nearly isoenergetic vibrational states. One state corresponds to pure out-of-plane bending of the planar NH3+ ion (0.60 electron volts), whereas the other state is a combination of in-plane and out-of-plane motion (0.63 electron volts). The product branching ratios differ markedly for these two vibrational-state preparations. The differences in reactivity suggest that the in-plane totally symmetric stretching mode is essentially inactive in controlling the branching ratio of this reaction.