High purity, highly ordered and twin free ortho-II phase YBa 2 Cu 3 O 6 . 5 0 crystals have been successfully prepared and their structural properties characterized by X-ray diffraction. Removal of twin boundaries was found to be essential in order to obtain highly ordered ortho-II crystals. The ortho-II phase shows a sharp superconducting transition at T c =62 K, 5 K higher than the ortho-I phase of the same oxygen content. The ordering was found to be three-dimensional with long correlation lengths ξ a =148 9, ξ b =430 9 and ξ c =58 9. No doubling of the unit lattice along the c-direction was observed, indicating an attractive interaction between oxygen ions in adjacent basal planes.