AbstractAn efficient method has been developed to follow the enzymic reactions involved in the biosynthesis of camphor inCinnamomum camphora(camphor tree) by use of [5,5‐2H2]geranyl diphosphate as a substrate and GC‐MS with selected ion monitoring. Borneol and camphor biosynthesized in the enzymic reactions gave a base peak due to the deuterium‐containing ion in its EI mass spectrum. It is possible to detect 1.5 ng of the biosynthesized borneol and camphor per injection into the GC‐MS. This method enabled us to differentiate easily the biosynthesized camphor from the endogenous camphor and it is a facile and sensitive technique to determine the amount of the biosynthesized