The phosphite triester (6), the O,O,O-thiophosphate triester (7), and the phosphate triester (8) of methyl ß-d-ribopyranoside were prepared. Crystal structures of 6 and 7 were determined by X-ray diffraction. Quantum-chemical calculations were performed on 6. They clearly demonstrate that the positive charge on phosphorus is relatively high. This high charge is a result of the strain in the ester moiety of 6.