The preparations of the new complexes [AsBr3{MeS(CH2)(2)SMe}], [ASX(3)([9]aneS(3))] (X = Cl, Br or I; [9]aneS(3) = 1,4,7-trithiacyclononane), [AsCl3([14]aneS(4))] ([14]aneS(4) = 1,4,8,11 -tetrathiacyclotetradecane), [AsX3([8]aneSe(2))] ([8]-aneSe(2) = 1,5-diselenacyclooctane), [(AsX3)(2)([16]aneSe(4))] ([16]aneSe(4) = 1,5,9,13-tetraselenacyclohexadecane), and [(AsBr3)(2)([24]aneSe(6))] ([24]aneSe(6) = 1,5,9,13,17,21-hexaselenacyclotetracosane) are described, These are obtained from direct reaction of the appropriate AsX3 and 1 mol equiv of the thio- or selenoether ligand in anhydrous CH2Cl2 (or thf for X = I) solution. The products have been characterized by microanalysis and IR and H-1 NMR spectroscopy. In solution they are extensively dissociated, reflecting the weak Lewis acidity of ASX3. Reaction of AsX3 with MeSe(CH2)(2)SeMe or MeC(CH2EMe)(3) (E = S or Se) gave only oils, Treatment of PCl3 or PBr3 with Me2S, MeE(CH2)(2)EMe, or [9]aneS(3) failed to give solid complexes, and there was no evidence from NMR spectroscopy for any adduct formation in solution. The crystal structures of the first series of thioether and selenoether complexes of As(III) are described: [AsBr3{MeS(CH2)(2)SMe}], C4H10AsBr3S2, a = 10.2818(6) Angstrom, b = 7.8014(5) Angstrom, c = 14.503(1) Angstrom, beta = 102.9330(2)degrees, monoclinic, P2(1)/c, Z = 4; [AsI3{MeS(CH2)(2)SMe}], C4H10AsI3S2, a = 9.1528(1) Angstrom, b = 11.5622(2) Angstrom, c = 12,0939(2) Angstrom, beta= 93,863(1)degrees, monoclinic, P2(1)/n, Z = 4; [AsCl3([9]aneS(3))b C6H12AsCl3S3, a = 17.520(4) Angstrom, b = 17.520(4) Angstrom, c = 16.790(7) Angstrom, tetragonal, /4(1)cd, Z = 16; [AsCl3([14]aneS(4))], C10H20AsCl3S4 a = 13.5942(2) Angstrom, b = 7.7007(1) Angstrom, c = 18.1270(3) Angstrom, beta = 111.1662(5)degrees, monoclinic, P2(1)/n, Z = 4; [(AsCl3)(2)([16]-aneSe(4))], C12H24As2Cl6Se4, a = 9.764(3) Angstrom, b = 13.164(1) Angstrom, c = 10.627(2) Angstrom, beta = 114,90(1)degrees, monoclinic, P2(1)/n, Z = 2; [(AsBr3)(2)([16]aneSe(4))], C12H24As2Br6Se4, a = 10.1220(1) Angstrom, b = 13.4494(2) Angstrom, c = 10.5125(2) Angstrom, beta = 113.49(2)degrees, monoclinic, P2(1)/n, Z = 2. [AsBr3{MeS(CH2)(2)SMe}] and [AsI3 MeS(CH2)(2)SMe}] reveal discrete mu(2)- halo As2X6 climeric structures involving distorted octahedral As(III), with the dithioether ligand chelating, [ASCl(3)([9]-aneS(3))] adopts a discrete molecular distorted octahedral geometry with the thioether behaving as a weakly coordinated fac-capping ligand, [AsCl3([14]aneS(4))] forms an infinite sheet involving two mu(2)-chloro ligands on each As but bridging to two distinct As centers. Each macrocycle coordinates to two adjacent As centers via one S atom, giving a cis-octahedral C14S2 donor set at As(III), The structures of [(AsCl3)(2)([16]aneSe(4))] and [(AsBr3)(2)([16]aneSe(4))] adopt 2-dimensional sheet structures with mu(2)-dihalo As2X6 dimers cross-linked by mu(4)-tetraselenoether macrocycles, giving a disorted cis-X4Se2 donor set at each As center. These species are compared with their antimony(III) and bismuth(III) analogues where appropriate.