5-Aminoimidazole-4-carboxamide-1-beta-D-[C-13(5)] ribofuranosyl 5-monophosphate ([C-13(5) ribose] AICAR-PO3H2) (6) has been synthesized from [C-13(5)]adenosine. Incorporation of the mass-label into [C-13(5) ribose] AICAR-PO3H2 provides a useful standard to aid in metabolite identification and quantification in monitoring metabolic pathways. A synthetic route to the C-13-labeled compound has not been previously reported. Our method employs a hybrid enzymatic, and chemical synthesis approach that applies an enzymatic conversion from adenosine to inosine followed by a ring-cleavage of the protected inosine. A direct phosphorylation of the resulting 2,3-isopropylidine acadesine (5) was developed to yield the title compound in 99% purity following ion exchange chromatography.