As2B10H8I2, M(r) = 519.8, monoclinic, I2/a, a = 14.262 (3), b = 8.128 (2), c = 21.575 (4) angstrom, beta = 92.65 (2)degrees, V = 2498 (2) angstrom 3, Z = 8, D(x) = 2.76 g cm-3, lambda(Mo K-alpha) = 0.71073 angstrom, mu = 101.8 cm-1, F(000) = 1840, T = 294 K, R = 0.036 for 2066 observed reflections. The structure contains a disordered distorted dodecahedron of As2B10 atoms with the As atoms adjacent. Two diiodinated (B-I) isomers (9,12- and 8,12-diiodo forms) are co-crystallized in the asymmetric unit, with the former predominating. The packing of the molecules in the crystal is determined by steric requirements of the adjacent exo-cage I atoms. Packing of the two isomers in the crystal lattice then results in apparent disorder of the two As atoms over four sites. Bond distances are As-As 2.435 (2) and 2.421 (2) angstrom respectively for the predominant forms of the 9,12- and 8,12-diiodo isomers. The B-I distances are 2.161 (7) and 2.170 (7) angstrom, and the mean B-B distance for boron positions which are not affected by arsenic disorder is 1.786 (11) angstrom.