A new procedure for the synthesis of the pyrophosphate bond has been employed in the preparation of nucleoside dipyrophosphates from nucleoside 3',5'-diphosphates. The method makes use of a powerful phosphorylating agent generated in a mixture of cyanoethyl phosphate, dicyclohexylcarbodiimide, and mesitylenesulfonyl chloride in order to avoid possible intramolecular reactions between the two phosphate groups on the sugar ring. That such reactions can readily occur was shown by the facile cyclization of deoxyguanosine 3',5'-diphosphate to P1,P2-deoxyguanosine 3',5'-cyclic pyrophosphate in the presence of dicyclohexylcarbodiimide alone. The phosphorylation reagent was initially tested in the conversion of deoxyguanosine 3',5'-diphosphate to the corresponding 3',5'-dipyrophosphate and was then used to phosphorylate 2'-O-(alpha-methoxyethyl)guanosine 3',5'-diphosphate, which had been prepared from 2'-O-(alpha-methoxyethyl)guanosine. In the latter case, the addition of the two beta phosphate groups was accomplished in 40% yield. Removal of the methoxyethyl group from the phosphorylated product gave guanosine 3',5'-dipyrophosphate, which was shown to be identical with guanosine tetraphosphate prepared enzymatically from a mixture of GDP and ATP. A modification of published procedures was also necessary to effect the synthesis of guanosine bis(methylenediphosphonate). Guanosine was treated with methylenediphosphonic acid and dicyclohexylcarbodiimide in the absence of added base. The product consisted of a mixture of guanosine 2',5' - and 3',5'-bis(methylenediphosphonate), which was resolved by anion-exchange chromatography. The 2',5' and 3',5' isomers are interconvertible at low pH, with the ultimate formation of an equilibrium mixture having a composition ratio of 2:3. The predominant constituent of this mixture has been unequivocally identified as the 3',5' isomer by synthesis from 2'-O-tetrahydropyranylguanosine.