The reaction mechanisms for the addition of azide anion to methyl 2,3-dideoxy-2,3-epimino-alpha-L-erythrofuranoside (I), methyl 2,3-anhydro-alpha-L-erythrofuranoside (II), and methyl 2,3-anhydro-beta-L-erythrofuranoside (III) were investigated with ab initio molecular orbital methods at the HF/3-21G level. A detailed characterization of the potential energy surface in vacuo allows us to localize stationary points and the possible reaction pathways. The solvent effects are discussed by means of a polarizable continuum model. The results indicate that the inclusion of solvent effects changes the order of stability of both products in I and II, and of both transition states in II. The results of III are qualitatively invariant in vacuo and in solvent. These findings suggest that a thermodynamic and kinetic controls take place in the addition process mechanism that would help explain the experimental data.