The substitution reactions of primary amines by tertiary phosphines in quadruply bonded dimolybdenum(II,II) complexes Mo2Cl4(NH2R)(4) have been studied. The exchange reaction has been shown to result at room temperature in disubstituted species Mo2Cl4(NH2R)(2)(PR3)(2) (PR3 = PMe3, NH2R = NH2Prn (1a), NH2But (2a), NH2Cy (3a); PR3 = PMe2Ph, NH2R = NH2Cy (4a)), while heating is needed to obtain fully substituted complexes Mo2Cl4-(PR3)(4) The crystal structure of disubstituted products has been investigated by X-ray crystallography and revealed that they all belong to the alpha-isomer, having both phosphine groups at the same Mo atom. Crystal data are as follows: for la, tetragonal space group I4(1)/a with a 17.737(2) Angstrom, c 15.6915(6) Angstrom, and Z = 8; for 3a, monoclinic space group P2(1) with a 10.963(3) Angstrom, b = 10.117(2) Angstrom, c 13.323(4) A, beta = 90.05(2)degrees, and Z = 2; for 4a, triclinic space group P1 with a = 9.329(3) Angstrom, b = 10.206(2) Angstrom, 18.975(3) Angstrom, a = 85.45(2)degrees, beta = 87.10(1)degrees, gamma = 80.88(1)degrees, and Z = 2. The substitution processes for the direct and reverse reactions have been monitored by P-31 NMR. They both proceed in a stepwise manner; however, a stereochemical hysteresis is taking place, i.e., the back reaction, the substitution of phosphines by amines, goes through another isomer of Mo2Cl4(NH2R)(2)(PR3)(2), having phosphine ligands on different Mo atoms. This beta-isomer is more thermodynamically stable and can be obtained by thermal conversion of the alpha-form. All chemical equilibria studied in the paper have been explained as governed by a higher trans effect of PR3 groups compared to NH2R groups.