A variable temperature high resolution 1H nuclear magnetic resonance study (at 300 or 500 MHz) of the two modified nucleosides 9-(2'-deoxy-ß-D-threo-ribofuranosyl)-adenine (1) and 9-(3'-deoxy-ß-D-threo-ribofuranosyl)-adenine (2) has been performed. It was found that the furanose conformation in 1 and 2 can be best described as a rapid North (N) South (S) equilibrium that is biased toward the N-form. For 1, a marked temperature dependence of the equilibrium was found, whereas the furanose conformation in 2 is virtually insensitive to temperature changes. Comparison of these results with the well-known conformational properties of the natural nucleosides 2'-deoxyadenosine (2'-dA) and 3'-deoxyadenosine (3'-dA) revealed that the net result of the gauche effect and the anomeric effect is of major importance in determining the furanose conformation. A brief discussion in terms of the thermodynamic parameters that govern the N S equilibria in the investigated nucleosides is given.