At high magnetic fields the 128.8 MHz ¹⁹⁵Pt NMR of all the species in the series [PtCl_n(H₂O)_6−n]⁴⁻ⁿ (n = 2–6) display unique ^35/37Cl isotope effects resulting in a unique ‘fine-structure’ of each individual resonance, which constitutes an unambiguous spectroscopic ‘fingerprint’ characteristic of the structure of the octahedral platinum(IV) complex, provided ¹⁹⁵Pt NMR are recorded at optimum magnetic field homogeneity and carefully controlled temperature (293 ± 0.1 K). The detailed ¹⁹⁵Pt resonance fine-structure observed experimentally can readily be accounted for by an isotopologue and isotopomer model for each complex, showing particularly noticeable differences between stereoisomer pairs such as the cis/trans- and fac/mer-complexes. Moreover partial isotopic ¹⁸O enrichment of the coordinated water molecules in the series [Pt^35/37Cl_n(H₂^16/18O)_6−n]ⁿ⁻² (n = 2–6) confirms this model. This technique can thus be considered a novel, direct spectroscopic method of chemical speciation of appropriate platinum(IV) complexes in solution without reference to accurate chemical shifts of authentic members of such a series. These effects are interpreted qualitatively in terms of the high sensitivity of ¹⁹⁵Pt NMR shielding to very small and subtle Pt–^35/37Cl and Pt–^16/18OH₂ bond displacements. Preliminary work shows this also applied to the corresponding bromido-complexes.