The first microporous framework structures containing uranium and chromium have been synthesized and characterized. Rb ₂[(UO ₂) ₂(CrO ₄) ₃(H ₂O) ₂](H ₂O) ₃ (1) was crystallized from uranyl chromate solution by evaporation. Further evaporation led to increased viscosity of the solution and overgrowing of Rb ₂[(UO ₂) ₂(CrO ₄) ₃(H ₂O)](H ₂O) (2) on the crystals of 1. With respect to 1, the framework of 2 is partially dehydrated. Both frameworks differ compositionally by only one water molecule, but this seemingly small difference affects significantly the pore size and overall structural topology of the frameworks, which present very different flexibility of the U-O-Cr links. These are rigid in the pillared framework of 1, in contrast to 2 where the U-O-Cr angles range from 126.3 to 168.2°, reflecting the substantial flexibility of Cr-O-U connections which make them comparable to the corresponding Mo-O-U links in uranyl molybdates.