AsX3 (X = Cl, Br or I) reacts with one molar equivalent of L [L = o-C6H4(PMe2)2, o-C6H4(PPh2)2, o-C6H4(AsMe2)2 or MeC(CH2AsMe2)3] in rigorously anhydrous CH2Cl2 or thf solution to give colourless or yellow solids of composition [AsX3L]. Using two molar equivalents of PMe3 with AsX3 affords solids of empirical formula [AsX3(PMe3)2], whereas one molar equivalent of PMe3 with AsCl3 gives the 1 ∶ 1 complex [AsCl3(PMe3)]. Reaction of AsX3 with AsMe3 gives only the 1 ∶ 1 product [AsX3(AsMe3)], whether using a 1 ∶ 1 or 1 ∶ 2 AsCl3 ∶ AsMe3 ratio. The products have been characterised by microanalysis, and IR, 1H NMR and 31P{1H} NMR spectroscopies, where appropriate. The crystal structures of three species, [AsCl3(PMe3)], [AsBr3{o-C6H4(AsMe2)2}] and [AsI3{o-C6H4(AsMe2)2}] have been determined. [AsCl3(PMe3)] adopts a dimeric structure formed through μ-dichloro-bridged edge-shared square pyramidal moieties with anti PMe3 ligands in the axial sites. Interestingly, in the lattice, there are two crystallographically independent dimers which are arranged orthogonally with long range As⋯Cl contacts between between them, giving rise to infinite chains. Thus, in one dimer the As centres are formally 5-coordinate, while in the adjacent dimer the coordination is [5 + 1]. The complexes [AsX3{o-C6H4(AsMe2)2}] (X = Br or I) are both dimeric through μ-dihalo-bridged bioctahedra in the isomer (A) form. Trends in bond length and angle distributions are considered and the stereochemical activity of the lone pair on the As(III) acceptor discussed.