Experimental observations show that for the gas phase isomerization of protonated molecules, a third body can transport a proton from a high-energy site to a lower energy-site of the substrate, thereby catalyzing the internal rearrangement. We examine the mechanism of isomerization of isoformyl cation to formyl cation using reactivity indexes defined in the context of a conceptual density functional theory. The analysis of the group charge capacity at the transition state reveals a proton push-pull effect between the catalyst and the C=O moiety of the substrate. This effect together with the nucleofugality of the catalyst drives the proton transfer catalysis. (C) 2008 Elsevier B.V. All rights reserved.