The experimental determination of the potential energy surface for gas-phase carbonyl addition-elimination reactions has been approached through the synthesis, isolation, and characterization of intermediates in four thermoneutral displacement reactions at carbonyl centers. The structure and reactivity of the putative intermediates were determined by both chemical and photochemical methods. For the reaction of Cl- + CH3OCOCl, the intermediates are shown conclusively to be unsymmetrical ion-molecule complexes, and the possibility of a stable, covalent tetrahedral adduct is ruled out. Infrared photoactivation of the intermediates results in unimolecular decomposition via an exothermic S(N)2 substitution channel at the ester position. For the reaction of Cl- with acetyl chloride, trimethylacetyl chloride, and trifluoroacetyl chloride, the tetrahedral species (intermediate or transition state) is observed to be energetically accessible under the experimental conditions. It is not possible, however, to determine unambiguously the structure(s) of the lowest energy species. The binding energies of the complexes and the existing kinetic and theoretical data, however, suggest that an electrostatic complex is most probable.