Polyhedral oligomeric silsesquioxanes (POSS) with an empirical formula [RSiO1.5], (where n = 6, 8.... 14, and R is a reactive organic group) represent a new type of nanomodifiers for polyurethanes (PUs) to produce organic-inorganic hybrids (OIHs), which are Promising candidates for modern applications spreading from biomedical to airspace technologies. In this paper we report on the synthesis and structural organization of two different series of PU-POSS copolymers (branched and cross-linked) based on a mixture of variable size oligomeric silsesquioxanes (POSS-M) of a general formula [(HOCH2-CH(OH)CH2)(2)N(CH2)(3)SiO1.5](n). Chemical structure of the synthesized POSS-M containing PUs has been confirmed by gel permeation chromatography and IR spectroscopic studies. Complex several-level hierarchy of their physical structural organization could be recognized from small angle X-ray scattering (SAXS) and atomic force Microscopic (AFM) studies. SAXS experiments revealed that processes of microphase segregation and formation of nano-sized domains enriched with the inorganic phase (POSS fragments) take place during synthesis of the OIH systems. These nano-heterogeneity regions contain up to 66 vol% of SiO1.5 and form a paracrystalline lattice with hexagonal symmetry and characteristic interplanar periodicity of around 15 nm. The average size of inorganic phase nano-inclusions can be estimated as 2-3 nm, which corresponds to several POSS moieties in size. Cross-linked POSS-M containing PUs, as topologically more complex systems, is characterized with a diffusion-limited microphase segregation features. In the latter case the regions of nano-heterogeneity are smaller in size, have smaller content of the inorganic phase (ca. 54 vol.%), and form a less perfect paracrystalline order with the periodicity of 9.7 nm. Generally, we demonstrate that replacement of regular POSS moieties with the mixed POSS-M ones can lead to nanocomposite systems with an ordered supramolecular structural organization and simultaneous reduction of their production cost due to relative simplicity of their synthesis. (C) 2008 Elsevier Ltd. All rights reserved.