The flexibility of pyrimidine nucleosides has been investigated by measuring their circular dichrotsm in hydvoalooholic solutions over a large temperature range (−100° to +40°C). It was observed that β-Uridine (Vrd) and B-Cytidine (Cyd) showed a decrease of the main diahroic band of about 40 percent, while sterically hindered nuoleoeides (α-Vrd, ara-Uracile, 02–2′ anhydro- Urd, 2′3′-O-isopropylidene-Urd) showed only small decreases. It is concluded that the flexibility of the glycosidic linkage in conjunction with the pseudo-rotation of the sugar residue is responsible for these changes; the thermodynamic values which can be deduced from these decreases are compatible with an oscillatory motion around the glycosidic bond, but exclude anti-syn transformations in pyrimidine nucleosides.