The insertion of arsenic into the base of the p-Rcalix[4]arenes (R = tert-butyl, H) via tris(dimethyla mino)arsine proceeds smoothly to give monoarsenic (5a, 5b) and diarsenic (6a, 6b) derivatives depending on whether the calix[4]arene:arsine ratio is 1:1 or 1:2, respectively (where a denotes R = tert-butyl and b, R = H). The monoarsenic species have the arsenic bonded to three of the four phenolic units of the calix[4]arene with the fourth containing a hydroxyl. The diarsenic species have two AsNMe(2) groups, each bonded to two adjacent phenolic units in a 1,2-alternate conformation. Tf moisture is present in the initial calix[4]arene starting materials, hydrolysis products (7a, 7b) are formed. These contain As-O-As bridges between the four phenolic units in a cone conformation. X-ray data : 5a, P (1) over bar, a = 9.821(2) Angstrom, b = 13.913(3) Angstrom, c = 16.884(4) Angstrom, alpha = 100.52(2)degrees, beta = 101.39(2)degrees, gamma = 90.23(2)degrees, Z = 2; 6b, P2(1)/n, a = 8.685(3) Angstrom, b = 10.482(3) Angstrom, c = 15.597(3) Angstrom, beta = 101.37(2)degrees, Z = 2; 7a, P2(1)/c, a = 13.067(4) Angstrom, b = 20.385(5) Angstrom, c = 18.652(5) Angstrom, beta = 96.90(2)degrees, Z = 4; 7b, P (1) over bar, a = 10.303(4) Angstrom, b = 11.175(3) Angstrom, c = 11.339(2) Angstrom, alpha = 117.12(2)degrees, beta = 102.29(2)degrees, gamma = 92.12(2)degrees, Z = 2.