Microspheres filled epoxy composites are fabricated, using two types of hollow glass microspheres, namely, Fillite‐500 and K‐15. Fillite‐500 microspheres are relatively stronger and denser (avg. crushed strength ∼17 MPa and density ∼0.45 kg/m3) than K‐15 microspheres (avg. crushed strength ∼2 MPa and density ∼0.15 kg/m3). Each type of microspheres in the range of 10–70 vol% are added into a low viscosity two‐part epoxy resin (SC‐15) system in several steps and are mixed together meticulously to avoid breakage of the microspheres during mixing. The epoxy/microspheres mixture is then cured at room temperature (about 70°F) inside an oven for 24 h followed by post curing at 200°F for 4 h. One of the objectives of this research is to determine the maximum amount of microspheres that can be mixed uniformly without aid of diluents. Microstructural examinations reveal that the microspheres are fairly distributed uniformly through out the epoxy matrix up to 70 vol% for Fillite‐500 and 60 vol% for K‐15. It is found that addition of microspheres results in the reduction of density of the neat epoxy up to 28% with Fillite‐500 and up to 54% with K‐15. Thermo mechanical analysis results show that the coefficient of thermal expansion of epoxy matrix is reduced up to 66% with Fillite‐500 and up to 51% with K‐15.Compressive behavior for the epoxy/Fillite‐500 system is found to be different than the epoxy/K‐15 system. Failure analysis results indicate that the failure processes are also different for both the systems. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers