The crystal structure of ceruleite, CuAl4[AsO4]2(OH)8(H2O)4, has been solved to an R1 of 0.0307, using the world's largest crystals from the Cap Garonne mine, France. Ceruleite crystallizes in space group P21/n, with the unit cell a = 7.2000(14), b = 11.345(2), c = 9.856(2) Å, β = 105.57(3)°, V = 775.6(3) Å3 and Z = 1. Ceruleite has a unique structure that consists of Al(O,OH)6 octahedra that are sharing edges to form rhombus-shaped tetramers. AsO4 tetrahedra share two corners with one such rhombus and the other two corners with each of two other rhombi, linking them into a very open mesoporous framework. Cu(OH)2(H2O)2 squares lie in the channels and link Al4 rhombi along || b. H2O molecules are also located in the channels.