The ECO2 elimination reactions of alkyl hydroperoxides proceed via abstraction of an α-hydrogen by a base: X− + R1R2HCOOH → HX + R1R2CO + HO−. Efficiencies and product distributions for the reactions of the hydroxide anion with methyl, ethyl, and tert-butyl hydroperoxides are studied in the gas phase. On the basis of experiments using three isotopic analogues, HO− + CH3OOH, HO− + CD3OOH, and H18O− + CH3OOH, the overall intrinsic reaction efficiency is determined to be 80% or greater. The ECO2 decomposition is facile for these methylperoxide reactions, and predominates over competing proton transfer at the hydroperoxide moiety. The CH3CH2OOH reaction displays a similar ECO2 reactivity, whereas proton transfer and the formation of HOO− are the exclusive pathways observed for (CH3)3COOH, which has no α-hydrogen. All results are consistent with the ECO2 mechanism, transition state structure, and reaction energy diagrams calculated using the hybrid density functional B3LYP approach. Isotope labeling for HO− + CH3OOH also reveals some interaction between H2O and HO− within the ECO2 product complex [H2O···CH2O···HO−]. There is little evidence, however, for the formation of the most exothermic products H2O + CH2(OH)O−, which would arise from nucleophilic condensation of CH2O and HO−. The results suggest that the product dynamics are not totally statistical but are rather direct after the ECO2 transition state. The larger HO− + CH3CH2OOH system displays more statistical behavior during complex dissociation.