High resolution NMR spectroscopy has been demonstrated to be capable of measuring the CO2/O2 specificity factor of ribulose 1,5-bisphosphate carboxylase/oxygenase. 31P-NMR provides a simple method for quantitatively determining the ratio of the products, 3-phosphoglycerate and phosphoglycolate. Both the specificity factor and degree of the reaction can be measured during the reaction without the need for complete consumption of ribulose bisphosphate or removal of it from the reaction mixture. Inorganic phosphate can also be detected and this may be used for monitoring the phosphatase activity and pH changes. Measurement by highly sensitive 1H-NMR is most time-efficient and is particularly suitable for the Rubisco with a high specificity factor. By optimizing the experimental conditions, it is possible to follow the simultaneous reactions in situ. The NMR method has been applied to three Rubisco enzymes with different values of specificity factor. Both 31P- and 1H-NMR gave similar results, agreeing with those previously reported by other methods.