The crystal and molecular structure of triethylammonium phenylthioarsenate, (C2H5)3NH +(C6H5)AsSO(OH)-, was determined from three-dimensional X-ray data collected with a manual diffractometer using Mo Ka ( 0.71069 k) radiation.The compound crystallizes in the triclinic space group PI, with a = 11.807 (1) Á, b = 10.558 (1) Á, c = 8.788 (1) A, a - 116.21 (2)°, ß = 98.95 (1)°, 7 = 117.91 (1)°, Z = 2, dmeosd = 1.40 (1) g cm"1 23, and d"i=d = 1.373 (3) g cm-3, at 23°. The structure was determined by Patterson and Fourier methods and refined with 362 observed reflections by full-matrix least squares to an R of 0.051. In the final refinement the phenyl ring was constrained as a rigid group, and the As and S atoms were refined with anisotropic thermal parameters. The anion of the structure consists of an arsenic atom tetrahedrally bonded to a phenyl group, two oxygen atoms, and a sulfur atom at distances of 1.90 (2), 1.65 (2), 1.75 (3), and 2.08 (2) Á, respectively. Hydrogen bonding holds pairs of anions together and one cation to each anion. This is the first monothioarsenate structure to be reported, and the As-S bond length is one of the shortest values reported for that bond.