The mixed ruthenium-copper oxide Sr(_2)YRu(_1)(_-x)Cu(_x)O(_6), is superconducting for low copper doping levels (x = 0.05 to 0.15), without the presence of cuprate planes, which are present in most high-temperature superconductors. Intriguingly the superconducting transition temperature and the Néel temperature are both ~ 30 K in this double perovskite, and thus below this temperature superconductivity and long-range magnetic order coexist, hi order to better understand the materials, both the crystal and magnetic structures need to be determined and this thesis concerns itself with this task through the utilisation of neutron and X-ray diffraction. This thesis begins with an introduction to the ruthenate double perovskites and examines particularly the necessary requirements for long-range magnetic order to establish itself in the material. An ordered crystal structure and an absence of competing magnetic interactions are prerequisites for magnetic ordering. This leads on to the copper doped ruthenates and the characterisation of the materials as superconductors. Initially the A(_2)YRu(_1-x)Cu(_x)O(_6) family, where A = Sr or Ba, is examined by neutron powder diffraction as yttrium is non-magnetic. This allowed the influence of the crystal structure on the magnetic structure of the ruthenium sublattice to be determined and its interactions revealed. The replacement of yttrium by a rare-earth element (e.g. Ho, Tb, Pr) has a profound effect on the magnetic interactions present in the material. Variable temperature neutron diffraction was vital, both for determining the magnetic structure and the interplay of these interactions in the material. The position of copper within this framework was examined, particularly by anomalous X-ray diffraction, and its important influence on the materials is discussed.