The geometry and energy profiles of the mutarotation pathway present in the equilibrium of 6-deoxy-β-l-mannopyranosyl 2,4-dinitrophenylhydrazine (1a), 6-deoxy-l-mannose 2,4-dinitrophenylhydrazone (1b), and 6-deoxy-α-l-mannopyranosyl 2,4-dinitrophenylhydrazine (1c) were modeled by DFT calculations at B3LYP/6-31G(d) level affording ΔG DFT =0.000kcal/mol, ΔG DFT =0.174kcal/mol, and ΔG DFT =3.411kcal/mol, respectively. Experimentally, the β-l-pyranose 1a occurs in 50% followed by the acyclic structure 1b in 44% as well as by the α-l-anomer 1c in 6%. The conformations of 1a–c and their corresponding 2,3,4-triacetyl derivatives 2a–c were studied by molecular modeling and NMR spectroscopy. IR frequencies, NMR chemical shifts, and X-ray diffraction analysis were employed to compare theoretical with experimental structural parameters.