The C4,-C5, conformation of the 5’-PIV model compounds 1-3 and the 5’-Pv TBP model compounds 4-6 has been determined in various solvents with 300- and 500-MHz ‘H NMR. It is found that lowering the solvent polarity results in a substantial increase of the g- populations for the model compounds 1, 2, 4, and 5 (e.g., for 1 in D20, x(g) = 0.00; CCI4, x(g-) = 0.23). This effect can be attributed to an enhanced electrostatic repulsion between the charge densities on Os and the endocyclic oxygen(s) at lower solvent polarities. This conclusion is supported by the experimental finding that the 5’-PIV system 3 and the 5’-Pv TBP system 6, in which the endocyclic oxygen(s) are replaced by C(H,), do not show a C4,-C5, conformational change when the medium polarity is changed. Furthermore, it is found that the 5’-PIV nucleotides 7 and 8, which are made soluble in apolar solvents by triesterification of the phosphorus, show also increased g- populations upon lowering the solvent polarity. The present results confirm our earlier proposal that enhanced charge repulsion between 05a,n d 01, in mononucleotides drives a rotation around the C 4 4 , bond toward g-.