Arsonic acids may be regarded as derived from orthoarsenic acid, by replacement of one of the hydroxyl groups with an organic residue, the arsenic acids are similarly derived by replacement of the two hydroxyl groups. The preparation of aromatic arsenic acids is described in this chapter. The most widely used is the Bart reaction, which involves the interaction of a diazonium salt with an inorganic arsenic compound. Arsenic acids are also obtained as by-products in the Bechamp synthesis. In the Rosenmund synthesis, salts of arsenic acids are obtained by treatment of aryl halides with sodium or potassium arsenite.