A new n.m.r. coupling constant torsion angle relation is utilized to explore the effect of exocyclic oxygen substituents at C-2′ and C-3′ on J1′2.′, J2′,3′, and J3′,4′ in the furanose ring of β-D-ribo-, β-D-arabino-, β-D-xylo-, and β-D-lyxonucleosides. The five vicinal couplings in β-D-deoxyribonucleosides are also investigated. Calculated coupling constants for the full pseudorotational itinerary at different values of the puckering amplitudes are given. It is shown that experimental coupling constants, taken from the literature, can be satisfactorily explained in all cases on the basis of a two-state (N/S)-model of the furanose pseudorotation. Good estimates of both N- and S- geometry and the conformational equilibrium constant in five-membered rings of ribose, deoxyribose, xylose, lyxose, and arabinose derivatives are given. The judicious use of geometrical information obtained by X-ray crystallographic studies appeared indispensable for this type of analysis.