Volume holography is a promising technology for high-capacity optical data storage. [1,2] When utilizing the entire volume of a medium instead of only its surface, capacities above one terabyte on a memory medium the size of a compact disc are theoretically possible. Recent developments in the field of holographic storage materials and advances in storage techniques and technology are summarized in review articles by Ashley et al. [3] and Hesselink et al. [4] Although there are still material challenges, write-once media seem to be fairly advanced. In search of rewritable media, many studies have been performed on photorefractive crystals and organic glasses. [1,4] Manufacturing single crystals of the required size is very expensive, and a major drawback of the organic photorefractive materials is the requirement of very high electric field strengths. With increasing material thickness, the voltage necessary for creating the field becomes difficult to handle. A different potential class of rewritable media are polymers containing photoaddressable azobenzene chromophores.[5] Many studies have been performed on azobenzene-containing polymer films with thicknesses in the range of 0.5-10 lm. [3,6] However, the preparation of the thick samples required for multiplexing with sufficiently low optical density remains difficult to achieve. Because the absorption coefficient depends on the wavelength, it is in principle possible to tune a laser with an adjustable wavelength (e.g., a dye laser) to the very edge of the absorption band. [7] If writing is to be performed at a wavelength where fixed-wavelength, solid-state lasers are available (515 or 532 nm), however, a concept that allows for diluting the chromophores is required. So far, the best results have been reported by Minabe et al. with a homogeneous polymer blend based on an azobenzene-containing polymer and a similar but nonabsorbing polymer. [8] In a sample of 250 lm thickness, a set of 20 data pages were successfully inscribed; yet, the individual holograms were not completely spatially overlapping. The important issues of stabilizing the inscribed data and rewritability were not addressed. We use a different concept of controlling the optical density, by using block copolymers that contain azobenzene side groups in one block. Block copolymers with less than 15 vol % of one component form spherical microphases with typical diameters in the range of 10-20 nm. Thus, we can macroscopically dilute the chromophores while, at the same time, keeping them in a local confinement. The latter is important for the stability of the inscribed information. In an earlier paper, we have already reported the formation of individual holographic gratings in 10 lm thick films of a material that contained azobenzene and nonabsorbing three-ring mesogenic side groups in the minority phase.[9] In the confined geometry, the presence of a cooperative effect of the side groups was demonstrated. The mesogenic side groups were reoriented together with the chromophores; furthermore, in materia...