The reactions catalysed by ribulosebisphosphate carboxylase/oxygenase have been studied by high resolution proton NMR spectroscopy. The protons of the substrate ribulose-P₂ and the products of the carboxylase and oxygenase reactions are readily assigned to the various carbon centres. Half of the 3-phosphoglycerate molecules generated during carboxylase turnover of ribulose-P₂ in saturating CO₂ concentrations acquire deuterium from the solution at the C₂ position, whereas the other 3-phosphoglycerate molecules and also the 3-phosphoglycerate produced as a result of the oxygenase activity contain no deuterium of this type. However, the addition of activated enzyme (i.e., in the presence of an effector) to a solution of ribulose-P₂ in the near absence of either CO₂ or O₂ catalyses the exchange of the C₃ proton with the deuterium of the solution.