Iron(II) bis(1,4,7-triazacylononane), Fe(tacn)(2)(2+), exhibits (1)A(1g) reversible arrow T-5(2g) spin-crossover behavior in solution. Values of DeltaH degrees (se) = 23 kJ mol(-1) and DeltaS degrees (se) = 67 J mol(-1) K-1 are obtained for the reaction by variable-temperature UV-visible and H-1 NMR spectroscopy. The thermodynamic parameters reflect intramolecular structural changes that accompany LS --> HS conversion and depend only to a small extent on solvent, counterion, and the presence of excess electrolyte. A procedure is described for quantitative correction of Fe(tacn)(2)(3+/2+) thermodynamics for the accompanying Fe(II) spin equilibrium.