Heating crystalline trans-[Co(NH₃)₄(¹⁵NH₃)SCN](N¹⁴CS)₂ in the dark at c. 60°C results in the following immediate products (partial formulae are shown): 39% trans-CoNCS²⁺ (without incorporation of ionic thiocyanate ), 31% cis-CoS¹⁴CN²⁺ and 30% cis-CoN¹⁴CS²⁺ (with incorporation of ionic thiocyanate ). A crystal structure of the unlabelled reactant [Co(NH₃)₅SCN](NCS)₂ [a 14.001(5), b 14.176(5), c 14.774(5) Ǻ, β 117.12(3)°, P2₁/n, Z = 8, 2898 reflections with I > 2σ(I), R 3.85%, Rw 4.10%] identifies two independent cobalt(III) cations in the asymmetric unit, and a consideration of the four distinct lattice thiocyanates identifies one of these, S(4)C(4)N(4)^-, as that most likely to be involved in replacing the originally coordinated SCN^-; S(4) at Co(1), and N(4) at Co(2). Intramolecular (as distinct from intermolecular) isomerization of coordinated thiocyanate is suggested for the formation of trans-CoNCS²⁺. The solid-state incorporation of lattice N¹⁴CS^- and S¹⁴CN^- is compared to that found in aqueous solution where ion-paired N¹⁴CS^- and S¹⁴CN^- as well as H₂O compete in a solvent-cage, intramolecular , exchange process. The structure of the coordination isomer [Co(NH₃)₅NCS]SO₄.1.5H₂O, containing the product CoNCS²⁺ cation [a 6.621(9), b 7.123(21), c 26.906(19) Ǻ, Pnma, Z = 4,524 reflections with I > 2σ(1), R 6.34%, Rw 7.67%], is reported. A reexamination of the crystal structure of [Co(NH₃)₅SCN]Cl₂.H₂O (ref. 1) suggests that hydrogen bonding to ammine ligands, rather than a more open structure or poorer positioning of lattice Cl^-, is the reason for the non-incorporation of ionic Cl^- on heating this material.