Proton magnetic resonance studies of 9-β-ᴅ-xylofuranosyladenine (xyloA), its 2′-amino-2′- deoxy, 2′-azido-2′-deoxy, 2′-bromo-2′-deoxy, 3′-thio-3′-deoxy, 3′-amino-3′-deoxy, 3′-azido-3′- deoxy, 3′-chloro-3′-deoxy, 3′-fluoro-3′-deoxy, 3′-O-methyl, 3′,5′-diazido-3′,5′-dideoxy analogues and 9-β-ᴅ-lyxofuranosyladenine (lyxoA) have been carried out to study the effect of chemical modifications at the sugar moiety on the solution conformational equilibria in these classes of nucleosides. Analogously to previous studies the xylose pucker can be described in the two-state N ⇔ S model of Altona and Sundaralingam. For the xylosides, however, a somewhat different N state (C3′-endo-C4′-exo) has to be used than for the ribosides and arabinosides (C2′-exo-C3′- endo). The results of the conformational analysis are that xyloA exists almost exclusively as an N conformed The effect of the substitutions in the modified compounds is to destabilize the N state. This decrease in the mole fraction of N is accompanied by an increase in the population of the g + rotamer of the exocylic 5′-CH 2 OD group. Thus for the xylosides a correlation N-t/g - or S- g + , respectively, can be derived. Proton relaxation rate measurements on 2′-azido-2′-deoxyxylo- furanosyladenine indicate that in the xylosides the standard syn or anti ranges do not represent stable positions for the adenine base, but that a glycosyl torsion angle (Χ∼160°, Υ∼80°) be tween syn and anti is preferred. LyxoA behaves similar to the xylosides and also favours the N state of the sugar pucker. In a summarizing discussion the conformational equilibria of the different modified pentosides - ribose, arabinose, xylose and lyxose - are compared. It is shown that generally intramolecular hydrogen bonding does not yield an important contribution to the stabilization of conformational equilibria in solution. It is also not possible to derive a quantitative relationship between such parameters as Van-der-Waals’ radii or electronegativity of the substituents and the mole fractions of the different conformers. It can, however, be seen that in all cases, where the hydroxyl groups at C2′ or C3′ are substituted by a more voluminous atom or group, steric effects become dominant and allow a qualitative explanation of the changes of the conformational equilibria. Only for the smallest and most electronegative substituents, like fluorine, other interactions (e.g. electrostatic) may become important. It is thus suggested that the purine(β)nucleoside conformation is essentially determined by steric interactions between the different parts of the molecule.