The photoluminescence and the local structure of sol−gel derived organic−inorganic hybrids, so-called ureasils, are discussed. Their host matrix is a silica-based network to which different numbers of oxyethylene repeat units8.5, 15.5, and 40.5 for U(600), U(900), and U(2000), respectivelyare covalently grafted by means of urea linkages. The small-angle X-ray scattering (SAXS) results suggest a diphasic structure for the morphology of the hybrids induced by local phase separation between siliceous domains and polymeric regions. The estimated interdomain distances, ranging from 27 A for U(600) to 59−64 A for U(2000), indicate that the three ureasils are greatly homogeneous on the SAXS scale. The luminescence spectra show a broad light emission (2.0−4.1 eV) with a blue band at ∼2.6 eV and a purplish-blue one at ∼2.8−3.0 eV, clearly distinguished by time-resolved spectroscopy. The energies of these two components are related to the dimension of the backbone inorganic skeleton. The local structure of these am...