Two 2D Anderson polyoxomolybdate‐based compounds, [{Ni(H2O)4}2{Ni(OH)6Mo6O18}]·4H2O (1) and [Ni(H2O)6][Ag2{Ni(OH)6Mo6O18}]·8H2O (2), were synthesized and characterized by elemental analysis, IR spectroscopy, SEM, and single‐crystal XRD. Their thermal behavior was studied by TGA, DSC, XRTD, and IR spectroscopy. Both 1 and 2 are arranged in 2D networks. In 1, [Ni(OH)6Mo6O18]4– Anderson units are covalently linked to each other by two different types of Ni2+, whereas in 2, the corresponding interactions are ensured exclusively by Ag+ ions. In addition, all the Ni2+ cations in 2 are located in interplanar positions and act selectively as counterions. Upon the addition of silver, the thermal stability of the Anderson anion is increased from 170 °C for pure nickel compound 1 up to 220 °C for mixed silver–nickel 2. Below 220 °C, 2 exhibits reversible hydration–dehydration processes with preservation of the Anderson anion structure. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)