Polymer- inorganic composite electrolytes very often show superior properties as compared to simple polymer electrolytes, and this enhancement is often ascribed to the structure and interfacial properties of the composite. Here, the structure of aluminosilica (AlSi) domains formed within organic -inorganic solid polymer electrolytes was studied using solid state Si-29 and Al-27 magic angle spinning (MAS) NMR, transmission electron microscopy (TEM), and nitrogen sorption experiments following material calcinations, to determine how the composite affects the properties in this class of polymer electrolytes. The major feature of all the calcined AlSi's based on 600 MW poly(ethylene glycol) (PEG) is the presence of two types of morphologies: nanoparticles with sizes of about 20-60 nm, and larger platelike particles. Increasing the amount of AlSi in the organic-inorganic composite material (OICM) increases the fraction of platelike particles relative to nanoparticles. The nanoparticles are practically nonporous, while the platelets are considerably mesoporous. When 100 000 MW poly(ethyelene oxide) is employed instead of low-molecular-weight PEG, the AlSi mainly consists of platelets with no porosity. The BET (Brunauer-Emmet-Teller) surface areas for all the samples are essentially equal to their external surface areas, indicating that pores located inside the AlSi are closed and do not participate in the Li conduction process. An increased fraction of AlSi nanoparticles vs platelets was found to provide higher interfacial surface area and also higher conductivity.