Five new heterometallic UO2 2+ -Co 2+ coordination polymers have been obtained by hydrothermal reactions of uranyl nitrate and metallic cobalt with aromatic polycarboxylic acids. Single-crystal X-ray diffraction reveals the formation of four 3D frameworks with the mellitate (noted mel) ligand and one 2D network with the isophthalate (noted iso) ligand. The compounds [(UO 2 (H 2 O)) 2 Co(H 2 O) 4 (mel)]·4H 2 O ( 1 ), [UO 2 Co(H 2 O) 4 (H 2 mel)]·2H 2 O ( 2 ), and [(UO 2 (H 2 O)) 2 Co(H 2 O)4(mel)] ( 4 ) consist of 3D frameworks built up from the connection of mellitate ligands and mononuclear metallic centers. These three compounds exhibit two types of geometry for the uranyl cation: pentagonal bipyramidal environment for 1 and 4 , and hexagonal bipyramidal environment for 2 . Using the mellitate ligand, the uranyl dinuclear unit is isolated in the compound [(UO 2 ) 2 (OH) 2 (Co(H 2 O) 4 ) 2 (mel)]·2H 2 O ( 3 ). Due to their 2D framework and the presence of uncoordinated cobalt(II) cations, the compound [(UO 2 )(iso) 3 ][Co(H 2 O) 6 ]·3(H 2 O) ( 5 ) is drastically different than the previous one. The thermal behavior of compounds 1 , 2 , and 3 has been studied by thermogravimetric analysis, X-ray thermodiffraction, and in situ infrared. By heating, the dehydration of compounds 1 and 2 promotes two structural transitions ( 1 → 1′ and 2 → 2′ ). The crystal structures of [(UO 2 (H 2 O)) 2 Co(H 2 O) 2 (mel)] ( 1′ ) and [(UO 2 )Co(H 2 mel)] ( 2′ ) were determined by single-crystal X-ray diffraction; each of them presents a heterometallic interaction between uranyl bond and the Co(II) center. Due to the rarely reported coordination environment for the cobalt center in compound 2′ (square pyramidal configuration), the magnetic properties and EPR characterizations of the compounds 2 and 2′ were also investigated.