The local chemical environments, dynamic heterogeneities, and nature of the interface in structured organic-inorganic composites prepared from poly(isoprene-b-ethyleneoxide) block copolymers, (3-glycidyl-oxypropyl)trimethoxysilane, and aluminum sec-butoxide via a sol-gel process are characterized by solid-state NMR. Such composites self-assemble into the characteristic morphologies of block copolymer systems, with the inorganic component selectively swelling the poly(ethylene oxide) phases. Here it is shown that the local chemical structure of the aluminosilicate component is nearly unperturbed by addition of the block copolymer and that the combined aluminosilicate/poly(ethylene oxide) layer is significantly le ss mobile than the polyisoprene. Spin-diffusion measurements on 8 composite with lamellar morphology indicate that no significant (>1 nm thick) poly(ethylene oxide) interphase exists between the polyisoprene and poly(ethylene oxide)/aluminosilicate layers, and therefore, the inorganic and poly(ethylene oxide) phases are intimately mixed on a molecular level. Implications of these findings for the materials properties are discussed.