High level ab initio calculations at the G2** level of theory have been used to examine the effect of interaction with a series of small neutral molecules (X = He, Ne, Ar, CO, HF, N-2, H2O, and NH3) on the barrier for rearrangement of the isoformyl cation (HOC+) to the formyl cation (HCO+). Interaction with species (He, Ne, and Ar) whose proton affinities are less than that of CO at oxygen leads to a reduction in the barrier from the value (147 kJ mol(-1)) in the isolated system, but the barrier remains positive. Interaction with molecules (HF and N-2) whose proton affinities lie between the proton affinities of CO at O and at C leads to the barrier becoming negative, thus allowing proton migration to take place without an overall barrier. Finally, interaction with molecules (H2O and NH3) whose proton affinities are greater than that of CO at C leads to a further lowering of the barrier; however, proton transfer to X rather than proton migration from O to C becomes energetically preferred. The most effective proton-transport catalysts for the rearrangement of HOC+ to HCO+ are thus molecules whose proton affinities lie between those of CO at O and at C.