Single crystal X-ray structure determinations are recorded for diphenyl(2-formylphenyl)phosphine gold(I) halides [Ph 2 (Ph-CHO)PAuX], X=Cl, Br and I, and for redeterminations of enhanced precision for triphenylphosphine gold(I) halides [Ph 3 PAuX], X=Cl, Br, I, and SCN 0.91 Br 0.09 . These complexes, other than [Ph 2 (Ph-CHO)PAuCl], together with a diverse array of other structures, crystallize as an isomorphous series in the orthorhombic space group P2 1 2 1 2 1 a=9.804(1)–11.906(3), b=11.771(2)–12.996(3) and c=12.871(1)–14.169(3)Å. In these complexes, introduction of the formyl group results in only minor differences between the conformations of the two phosphine ligands and the corresponding Au–P, Au–X, and Au–P–X bond lengths and angles. The crystal packings of [Ph 3 PAuX] for X=Cl, Br, I and of [Ph 2 (Ph-CHO)PAuX] for X=Br and I show that, while these structures are isomorphous, different supramolecular synthons may be present, suggesting global packing considerations are all-important rather than specific supramolecular interactions. This is borne out by the different packing found for the centrosymmetric [Ph 2 (Ph-CHO)PAuCl] structure. Crystallization of the mixed anion structure [Ph 3 PAuSCN 0.91 Br 0.09 ] in the above P2 1 2 1 2 1 lattice rather than the P2 1 /c lattice reported for pure [Ph 3 PAuSCN] suggests that co-crystallization with bromide may impose constraints on packing considerations which favor crystallization in the P2 1 2 1 2 1 lattice.