High-level ab initio calculations using the G2** and G2(ZPE=MP2) methods have been employed to examine the effect of interaction with a range of neutral molecules (X) on the barrier to the degenerate proton-transport reactions in NHH+ (X = Ar, HF, CO, N-2, H2O) and methyl cation-transport reactions in NNCH3+ (X = HF, H-2, N-2, HCl, H2O). It is found that the barriers to both proton and methyl cation transport are lowered from their values in the isolated ions o 182 and 152 kJ mol(-1), respectively, by interaction with species having values of the proton or methyl cation affinity, respectively, lower than that of molecular nitrogen. Interaction with species that have larger values of proton or methyl cation affinities leads to a further lowering of the barrier, but transfer to the neutral molecule X becomes the more energetically favorable process in such cases. It is found that the ideal catalyst for ion transport should have an ion affinity close to but less than that of molecular nitrogen and have a large dipole moment.