Monomeric two-coordinate gold(I) complexes, [Au(P(mes)3)X] (P(mes)3 = tris(2,4,6-trimethylphenyl)phosphine, X = Cl, Br and I), have been prepared and characterized by single-crystal X-ray structure determinations, far-infrared spectroscopy and solution and solid-state CPMAS 31 P n.m.r. spectroscopy. X-Ray structure determinations show that crystals obtained from solutions of [NBu4] [AuX2] and P(mes)3 in acetonitrile for X = Cl, Br and I and in dimethylformamide (dmf) for X = Br and I form an isomorphous series of complexes, crystallizing in space group P21/c with a ꠸, b ꠲2, c ꠱3 Ŭ b ꠹8ࠨa form). Crystallization of the chloride from dimethylformamide yields the solvated complex [Au(P(mes)3)X]稤mf) in space group P2/a with a 15.224(2), b 10.070(1), c 18.210(4) Ŭ b 100.42(2)஠Electrochemical synthesis of the complexes for X = Cl and Br yield two new crystalline phases; the chloride in space group P21/c with a 10.249(2), b 8.189(2), c 31.844(3) Ŭ b 91.68(1)ࠨb form) and the bromide in space group Pbca with a 19.208(4), b 15.586(3), c 16.962(4) Š( g form). The Au-P bond lengths increase in the order Cl < Br < I with distances c. 0.02-0.03 Šlonger than average values for other [Au(PR3)X] complexes, reflecting steric congestion by the P(mes)3 ligand. For the unsolvated complexes, the Au-X distances are c. 0.02 Šshorter than average values. For the Cl/dmf solvate, both Au-P and Au-X bond lengths increase. For the a complexes, far-infrared spectra show n(Au 35,37 Cl) 336, 329 cm -1 , n(AuBr) 234 cm -1 and n(AuI) 195 cm -1 and solid-state 31 P CPMAS n.m.r. spectra yield broad peaks with d-3.9 (Cl), -0.6 (Br) and +6.0 I). For the Cl/dmf solvate, n(Au 35,37 Cl) are 334, 327 cm -1 and d is -4.4. Solution 31 P n.m.r. spectra in CDCl3 give sharp single peaks at d -5.0 (Cl), -1.4 (Br) and +5.5 (I) with the similarity of the values with those for the solid-state spectra consistent with similar conformational structures for the [Au(P(mes)3)X] molecules in the two states.