The reaction of cis-Re2(μ-O2CMe)2Cl4(H2O)2 1 with P(C6H4OMe-p)3 gives the tetranuclear complex Re4(μ-O)4Cl4[P(C6H4OMe-p)3]4 14 along with the salt [(C6H4OMe-p)3PMe]2Re2Cl8 15. Compound 14 is the first symmetrical, neutral, tetrarheniumcyclodiyne type cluster that contains phosphine ligands. Complexes of this same type with PPh3 (16) and PMe2Ph (17) have been prepared by alternative methods but have poor solubility properties. The paramagnetic complex Re2(μ-O2CMe)Cl4(PPh3)2 2, which is prepared from 1 by reaction with PPh3, undergoes phosphine substitution reactions when treated with dichloromethane solutions of other phosphines. Simple non-redox reactions can occur upon reaction of 2 with monodentate and bridging bidentate phosphines to afford complexes of the types Re2(μ-O2CMe)Cl4(PR3)2 [PR3 = PBz3 3, P(C6H4OMe-p)3 4 or PMePh2 5] and trans-Re2(μ-O2CMe)Cl4(μ-PP)2 [PP = dppm 9, Ph2PNHPPh2 10 or (Ph2P)2CCH2 11]. However, reactions that lead either to reduction to dirhenium(II) complexes or Re–Re bond cleavage are encountered in the case of the reactions of 2 with PMePh2, PMe2Ph, PCy3 and dppe; these have led to the isolation of Re2Cl4(PMePh2)4 6, Re2Cl4(PMe2Ph)4 7, mer-trans-ReOCl3(PCy3)2 8, α-Re2Cl4(dppe)2 12, and trans-[ReO2(dppe)2]Cl 13. The structures of compounds 3, 8, 13, 14 and 15 have been determined by X-ray crystallography.