Using regiospecifically N-15-labeled complexes, the N-1 (adjacent-Me) to N-2 (remote-Me) linkage isomerization reaction of (5-methyltetrazolato)pentaamminecobalt(III) has been shown to be intramolecular in Me2SO and H2O as solvents. The results exclude a ligand dissociation and reentry mechanism or rearrangement via a symmetrical eta5 intermediate; an eta2 pi-bonded tetrazolato intermediate is suggested. A degenerate remote-Me to remote-Me rearrangement (N-2 to N-3) has also been identified through the labeling experiments and shown to be intramolecular also. It is slower (25-fold) than the N-1 to N-2 rearrangement, as might be anticipated because the Me interaction with the bound NH3 groups is diminished in the remote configuration. The N-1 isomer of the uncharged tetrazole ligand is some 100-fold faster to rearrange than its deprotonated form, but N-15 experiments show that this rearrangement is also intramolecular and does not proceed via an eta5 pi-bonded intermediate or through dissociation and reentry of the ligand. A degenerate (and 10-fold slower) N-2 to N-3 rearrangement has also been identified for the uncharged ligand complex. Degenerate N-1 to N-4 rearrangements were not competitive with the N-1 to N-2 processes for either the anionic or uncharged ligand complexes. The large upfield shifts for the N-15 signals and in particular the strong negative NOEs observed in the proton-decoupled spectra establish N4 as the site of protonation for the N-1 and N-2 isomers; the tautomers involved have not been established previously. Faster rearrangement for the more weakly bound neutral ligand complexes is consistent with an essentially dissociative, although still intramolecular, rearrangement process.