Four heterobimetallic U(VI)/M(II) (M = Mn, Co, Cd) carboxyphosphonates have been synthesized. M2[(UO2)6(PO3CH2CO2)3O3(OH)(H2O)2]·16H2O (M = Mn(II), Co(II), and Cd(II)) adopt cubic three-dimensional network structures with large cavities approximately 16 Å in diameter that are filled with co-crystallized water molecules. [Cd3(UO2)6(PO3CH2CO2)6(H2O)13]·6H2O forms a rhombohedral channel structure with hydrated Cd(II) within the channels. The cubic compound (Co) displays differential gas absorption with a surface area for CO2 uptake of 40 m2 g−1 at 273 K, and no uptake of N2 at 77 K.