The complex {Fe[HC(3,5-Me(2)pz)(3)](2)}(BF4)(2) (pz = pyrazolyl ring) undergoes a phase transition that occurs concomitantly with a thermally induced spin conversion between the high-spin (HS. S = 2) and low-spin (LS, S = 0) states. Above 204 K the compound is completely HS with the structure in the C2/c space group with Z = 4. A crystal structure determination of this phase was performed at 220 K yielding the cell constants a = 20.338(2) Angstrom, b = 10.332(1) Angstrom, c = 19.644(2) Angstrom, beta = 111.097(2)degrees, and V = 3851.5(6) Angstrom (3). There is one unique iron(II) site at this temperature. Below 206 K the compound converts to a 50:50 mixture of HS and LS, The radical change in the coordination sphere for half of the iron(II) sites, most notably a shortening of the Fe-N bond distances by ca. 0.2 Angstrom, that accompanies this magnetic transition causes a phase transition. The crystal system changes from C-centered monoclinic to primitive triclinic with Z = 2 with two half-molecules on independent inversion centers. A crystal structure determination was performed at 173 K in space group P (1) over bar with a 10.287(2) Angstrom, b = 11.355(3) Angstrom, c 18.949(4) Angstrom, alpha = 90.852(4)degrees, beta = 105.245(4)degrees, gamma = 116.304(4)degrees, and V = 1892.3(8) Angstrom (3). All specimens investigated below the phase transition temperature were determined to be nonmerohedral twins. Temperature cycling between these two forms does not appear to degrade crystal quality. previous magnetic susceptibility measurements indicate a second, irreversible increase in the magnetic moment the first time the crystals are cooled below 85 K. A crystal structure determination at 220 K of a specimen precooled to 78 K was not significantly different from those not cooled below 220 K.