This volume contains the hydrous oxo compounds of the metals Sb to Cr (System Nos. 18 to 52) with molybdenum. (The corresponding anhydrous compounds have been described in the volumes "Molybdan" Erg.-Bd. B 1 and B 2.) With these metals, molybdenum forms monomolyb- dates, isopolymolybdates, peroxomolybdates, and molybdometalates. Antimony forms only the compound H3SbM01204o' n H0 and with bismuth no hydrous oxo 2 compounds are known. More than half the volume describes the monomolybdates, isopolymolybdates, and peroxo- molybdates of the alkali and alkaline earth metals, including ammonium and organic cations. First, a detailed review on the structures, spectra, and other properties of the various iso- polymolybdate types is given. It also contains a comprehensive description on photochromism, a property characteristic of many of the organic ammonium polymolybdates. Then follows the description of the individual alkali compounds. The large group of molybdates with organic cations is placed after the caesium molybdates. The photochromism and the photogalvanic effect are investigated at length for (i-C3H7NH3)6[M07024)' 3 H0. The alkaline earth molybdates 2 are treated briefly compared with the alkali molybdates, e. g., no new data are available on beryllium molybdates.