The structures of three temperature-dependent polymorphs of solvent-free decamethylferrocenium tetracyanoethenide, [FeCP*(2)][TCNE], are determined from high-resolution synchrotron powder diffraction data. [FeCP*(2)][TCNE] is the first organic-based ferromagnetic material to be synthesized and is known to have two structural phase transitions at 249 and 282 K. The low-temperature phase, which exhibits spontaneous ferromagnetic order below 4.8 K, was determined at 12 K. At that temperature, it has monoclinic space group P2(1)/c [a = 9.6637(4) angstrom, b = 14.1217(5) angstrom, c = 18.6256(7) angstrom, beta = 113.231 (2)degrees, Z = 41 and consists of parallel chains of alternating [Fe(C5Me5)(2)](center dot+) and [TCNE](center dot-) ions, with an intrachain Fe center dot center dot center dot Fe distance of 10.45 angstrom. Structures of the intermediate and ambient temperature phases, also studied here, are characterized by increasing disorder. At 250 K, the unit cell space group is P2(1)/m [a = 9.7100(3) angstrom, b = 14,4926(4) angstrom, c = 9.4997(3) angstrom, beta = 113.153(1)degrees, Z = 2]. At ambient temperature, the lattice, albeit quite disordered, belongs to the orthorhombic space group Cmcm [a = 10.629(1) angstrom, b = 16.128(1) angstrom, c = 14.593(1) angstrom, Z = 4]. Nearest-neighbor magnetic interactions were evaluated for the 12 K structure by CASSCF and CASSCF/MCQDPT calculations (a methodology similar to the CASPT2 method). Similar trends are observed in computations with and without inclusion of spin-orbit coupling. The strongest are two intrachain [FeCP*(2)](center dot-)center dot center dot center dot[TCNE](center dot-) interactions (ferromagnetic with values of similar to 45 and similar to 29 cm(-1)), although weaker, nonnegligible, ferro- or antiferromagnetic interchain interactions of less than +/- 0.2 cm(-1) are also present. Magnetic interactions that lead to ordering are therefore three-dimensional, despite the vastly different intra- and interchain coupling strengths.