The discrete inverted-Keggin ion [Mo12O32(OH)2(4-N3C2H2-C6H4AsO3)4]2- (1) has been prepared in an aqueous acidic (pH 0.8) medium by the reaction of MoO3 with the (4-triazolylphenyl)arsonic acid 4-N3C2H2-C6H4AsO3H2 under hydrothermal conditions and was isolated as a sodium salt in 21% yield. The exact same reaction in the presence of Cu2+ ions resulted in the neutral metal-organic framework (MOF) Cu2[Mo12O34(4-N3C2H2-C6H4AsO3)4] (Cu-1) in 68% yield. The inverted-Keggin ion 1 comprises a metal-oxo core, which is capped by four organoarsonate groups, and in Cu-1, individual polyanions are linked in the solid state by coordination of the Cu2+ ions with the triazolyl groups. The discrete ion 1 was characterized by single-crystal X-ray diffraction (XRD), Fourier transform infrared (FT-IR), and atomic absorption (AA) spectroscopy, as well as thermogravimetric analysis (TGA), and the POM-MOF Cu-1 was characterized by single-crystal and powder XRD, FT-IR, TGA, and gas sorption. Cu-1 has channels with a diameter of around ∼0.9 nm and exhibits a water-vapor adsorption capacity of 89.7 cm3 g-1 (p/p0 = 0.95).