Detailed spectroscopic data have been obtained for arsonoacetic acid, As(CH₂COOH)O₃H₂, and its barium and sodium salts. The X-ray crystal structure of the free acid is isomorphous with phosphonoacetic acid. Reduction gave the As(I) compound arsenoacetic acid, (AsCH₂COOH)n which was shown by ESI-MS to contain cyclic species based on As–As bonds, with n mainly 3–6. The X-ray crystal structure of the hexamer was determined as the pyridine solvate and shown to have a hexacyclic As₆ ring in a puckered chair conformation, with –CH₂COOH groups in equatorial sites, each H-bonded to a pyridine molecule in the lattice.