Conformational study on phosphopantetheine shows that this compound has an intrinsic tendency to adopt a multitude of conformations which contain hydrogen bonds involving the sulphydryl, hydroxyl, carbonyl and amide groups. The sulphydryl group may form a hydrogen bond with the C(7') = 0 carbonyl group, the latter being also involved in hydrogen bonding with the N(4')-H GROUP. All these hydrogen bondings occur for different conformations around the backbone. The N(7')-H and C(4') = 0 groups are not involved in hydrogen bonding. It is also found that a strong interaction occurs between N(4')-H and 0-3' which is responsible for a rigid conformation around the C(3')-C(4') and C(3')-0(3') bonds. As far as the phosphate group is concerned the results show that this group may interact with the 0(3')-H hydroxyl group to form hydrogen-bonded rings of different sizes. A six-membered ring formed by hydrogen bonding between 0(3')-H and 0-1' appears more favorable than an eight-membered ring involving an anionic oxygen instead of an ester oxygen related to the phosphate group.