Single crystals of two new uranyl selenates K₃(H ₃O)[(UO₂)4(SeO₄)6(H₂O)4] ·5H₂O (I) and K2.5(NO3)0.5[(UO₂) ₂(SeO₄)₃ (H₂O)]·4H ₂O (II) have been prepared by room-temperature evaporation from aqueous solution of uranyl nitrate, selenic acid, potassium carbonate and (for the compound I) carbamide. The crystal structure of I has been solved by direct methods [monoclinic, P21/m, a = 12.001(3), b = 13.613(3), c = 13.753(3) Å, β = 109.187(4)°, V = 2122.0(8) Å3 and Z = 2] and refined to R₁ = 0.029 (wR2 = 0.084) for 4865 reflections with |Fo| = 4sF using least-square methods. The crystal structure of II has been solved by direct methods [monoclinic, ?2/?, a = 20.290(4), b = 10.380(2), c = 21.436(4) Å, β = 103.446(3)°, V = 4391.0(13) Å3 and Z = 4] and refined to R₁ = 0.027 (wR2 = 0.066) for 7944 reflections with |Fo| = 4sF using least-square techniques. The structures of I and II are based upon the [(UO ₂)₂(SeO₄)₃(H₂O) ₂]2- and [(UO₂)₂(SeO₄) ₃(H₂O)]2- layers, respectively, consisting of UO7 pentagonal bipyramids sharing corners with SeO₄ tetrahedra. Potassium cations induce curvature of the uranyl selenate layers, which is mediated by the interlayer water molecules, hydronium ions and nitrate groups. The topology of the 2D units in the structure of I is novel for the structural chemistry of uranyl selenates.