The potential energy surface of the gas-phase reaction between halide ions (F- and Cl-) and methyl formate has been investigated by ab initio calculations. For F-, two pathways have been observed at thermal energies and identified in the calculations: (1) a-elimination of CO to yield a fluoride-methanol adduct, the so-called Riveros reaction that has found wide application in gas-phase ion chemistry, and (2) S(N)2 displacement of HCOO-. The first reaction is shown to proceed by the initial formation of a loose complex followed by formal abstraction of a formyl hydrogen to yield a three-body complex that dissociates into the final products. The S(N)2 reaction initially involves formation of a loose complex with the fluoride attached to the methyl group of the ester. The first pathway is calculated to go through a lower energy local transition state than the corresponding S(N)2 reaction but the transition states are located below the energy of the reagents. Both ion-neutral complexes can interconvert via formation of a stable tetrahedral intermediate. The product distribution was estimated via a simple RRKM calculation that predicts 92% of alpha-elimination and 8% Of S(N)2 reaction. This prediction is in excellent agreement with measurements carried out by FT-ICR. This product distribution is predicted to remain essentially unchanged for the reaction with DCOOCH3 in agreement with experimental observations. A similar analysis of the corresponding Cl- + HCOOCH3 reaction reveals that a-elimination has a substantial activation energy (well above the reagents) accounting for the failure to observe this reaction even though it is exothermic. These calculations also reveal that for the Cl- system, the tetrahedral intermediate is not a stable intermediate in agreement with previous experimental data on related systems.