The gas phase ion-molecule reaction (RCOOR′)H+ + (CH3)3CCl → RC(OR′)(OC4H9)+ + HCl has been investigated for a number of esters (R, R′ = alkyl) by ion cyclotron resonance spectroscopy. This reaction proceeds with sizable rate constants for alkyl formates and methyl chloroacetate, i.e., k = (5.2 ± 1.5) × 10-10 cm3 molecule-1 s-1 for methyl formate. These observations are rationalized in terms of a mechanism in which proton transfer from the ester to the halide and the ease with which a nascent carbonium ion can be generated in the collision complex are proposed to be responsible for the efficiency of these processes. © 1979 American Chemical Society.831114881491