The irreversible reaction of methyl triflate with the neutral Re(I) tetrazolato complexes of the type fac-[Re(diim)(CO)3(L)], where diim is either 1,10-phenanthroline or 2,2’-bipyridine and L is a para substituted 5-aryltetrazolate, yielded the corresponding cationic methylated complexes. While methylation occurred regioselectively at the position N4 of the tetrazole ring, the cationic complexes were found to exist in solution as an equilibrating mixture of linkage isomers, where the Re(I) centre was bound to either the N1 or N2 atom of the tetrazole ring. The existence of these isomers was highlighted both by NMR and X-ray crystallography studies. On the other hand, the two isomers resulted indistinguishable by means of IR, UV-Vis and luminescence spectroscopy. The prepared cationic complexes are all brightly phosphorescent in fluid and rigid solutions, with emission originating from triplet metal-to-ligand charge transfer excited states. Compared to their neutral precursors, which are emitting from admixtures of triplet metal-to-ligand and ligand-to-ligand charge transfer states, the methylated complexes exhibit blue-shifted emission characterised by elongated excited state lifetimes and increased quantum yields. The nature of the excited states for both the neutral and methylated complexes was probed by means of Raman and transient Raman spectroscopy and with the aid of Time-Dependent Density Functional Theory calculations. Lastly, both the neutral and methylated species were used as emitting phosphors in the fabrication or Organic Light Emitting Devices and Light Emitting Electrochemical Cells.