Twenty molybdoarsenic acid and salts MxAs2Mo18O62.nH2O (M = NH4+, Li+, Na+, K+, Cs+, Ca2+, Mg2+, Sr2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Al3+, La3+, Ce3+, Pr3+, Nd3+, Sm3+, Gd3+) with Dawson structure, were synthesized and characterized by IR and UV spectroscopy, polarog., cyclic voltammetry and thermogravimetry. Hexa-basicity were established using conductive and acid-base titrns. The reduction mechanism of As2Mo18O626- in aqueous solution is suggested. The UV bands of the compounds were assignedf as e ?e* and e ?b2 transition according to the energy levels of MO's of MOL5 type compounds with C4v symmetry. The effects of the countercations Mn+ on the IR spectra, thermal decomposition temperature and decomposition products are discussed.

Key words: ULTRAVIOLET SPECTROPHOTOMETRY, OXIDATION REDUCTION REACTION, REACTION MECHANISM, INFRARED SPECTROPHOTOMETRY, HETEROPOLYACID, MOLYBDATE, ELEMENTAL ANALYSIS, EXTRACTION, POLAROGRAPHIC ANALYSIS, THERMOGRAVIMETRY