The relative gas-phase acidity of halosubstituted acetic acids CH2XCO2H, CHX2CO2H and CX2CO2H (X = F, Cl, and Br) is analyzed in terms of global and local descriptors of reactivity. The model is based on the analysis of proton-transfer equilibria with reference to acetic acid CH3CO2H. The relative acidity pattern displayed by this series is rationalized in terms of the hard and soft acids and bases principle. The relative stability between the neutral species and the corresponding anions is in agreement with the maximum hardness principle. Charge transfer between the conjugated bases present in the proton-transfer equilibria is correctly accounted for by using a classical ion transport model that introduces the electronic chemical potential of transfer. The local reactivity analysis based on regional Fukui functions and local softness displays a good correlation with the experimental gas-phase acidity within the series.