The tetramer [Fe4S4(S-t-Bu)4]2− in acetonitrile solution at ambient temperature undergoes facile ligand substitution reactions with thiols R′SH yielding [Fe4S4(S-t-Bu)4−n(SR′)n]2−. The reactions have been monitored by electronic spectral and pmr studies of equilibrium solutions. In the cases of R′ = p-C6H4NMe2 and p-tolyl the n = 1−4 species have been detected in the contact-shifted methyl pmr spectra and equilibrium constants for ligand exchange between two such species approach statistical values. The following ligand substitution series was established: MeC(O)SH ∼ p·XC6H4SH ≳ Ac-l-Cys-NHMe ≳ p-YC6H4SH >PhCH 2SH > HOCH2CH2SH > EtSH ⪢ p·MeC6H4OH (X = H, NO2, Me; Y = NMe2, NMe3+). Substitution tendencies roughly parallel aqueous acidities, at least to pKa≳ 6.5, and aryl thiols effect full substitution at (R′SH)/(tetramer) mole ratios of 4.5−4.9. The structure of a representative aryl-substituted tetramer, (Me4N)2[Fe4S4(SPh)4], has been determined from 2844 independent reflections collected by counter methods. The red-black crystals are orthorhombic, space group D24-P212121, with a = 11.704 (11), b = 23.944 (16), and c = 14.876 (10) Å and a calculated density of 1.492 g/cm3 for four formula units in the unit cell. The structure consists of discrete cations and anions. The Fe4S4* core, as that of [Fe4S4(SCH2Ph)4]2−, has effective D2d symmetry. The principal difference between the structures of the two tetramers occurs in the detailed geometry of the Fe4 portion, which in [Fe4S4(SPh)4]2− more closely approaches Td symmetry with an average Fe …Fe distance of 2.736 Å. The bonded Fe-S* distances occur as sets of four (2.267 (5) Å) and eight (2.296 (4) Å), giving an average of 2.286 Å. The average terminal Fe-S distance is 2.263Å. Comparison of the structures of the two tetramers does not provide any clear indication that [Fe4S4(SPh)4]2− is inherently the more stable. The positions of the ligand substitution equilibria are concluded to be dominated by the acid-base characteristics of tert-butylthiolate and R'SH. The utility of the ligand substitution reactions in synthesis is illustrated by the preparation and isolation of salts of [Fe4S4(SPh)4]2− and [Fe4S4(SePh)4]2− by reaction of [Fe4S4(S-t-Bu)4]2− with benzenethiol and diphenyl diselenide, respectively, in acetonitrile solution.