Two-dimensional X-ray diffraction (XRD) analyses of transparent mesoscopically ordered silica/ block copolymer composite monoliths reveal single-crystal-like patterns that correspond to well-ordered hexagonal domains that are greater than 1 cm x 1 mm x 1 mm in size. Analyses of the diffraction patterns reveal highly uniform hexagonal domains with narrow distributions of orientational order that are characterized by a phi-mosaic smaller than 2 degrees in the plane of the hexagonal lattice and a chi-mosaic smaller than 11 degrees along the cylindrical aggregate axes. XRD and TEM results also show that the composite monoliths have large well-ordered cubic mesophase domains in the meniscus regions of the samples. Boundaries between the hexagonal and cubic domains in the monoliths reflect gradual transitions between the two epitaxially related structures. Such large single-domain hexagonal and cubic mesostructures were furthermore preserved following removal of the organic species by calcination to produce mesoporous silica monoliths.