The N-15-labeled diammine(mu-oxo) ruthenium complex cis,cis-[(bpy)(2)((H3N)-N-15)(RuORuIII)-O-III((NH3)-N-15)(bpy)(2)](4+) ((2-N-15)(4+)) was synthesized from cis,cis-[(bpy)(2)(H2O)(RuORuIII)-O-III(H2O)(bpy)(2)](4+) by using ((NH4)-N-15)(2)SO4 and isolated as its perchlorate salt in 17% yield. A 1:1 mixture of (2-N-15)(4+) and nonlabeled cis,cis-[(bpy)(2)((H3N)-N-14)(RuORUIII)-O-III((NH3)-N-14)(bpy)(2)](4+) were electrochemically oxidized in aqueous solution. The gaseous products N-14(2) and N-15(2) were formed in equimolar amounts with only a small amount of (NN)-N-14-N-15 detected. This demonstrates that dinitrogen formation by oxidation of the diammine complex proceeds by intramolecular N---N coupling.