AbstractThree‐phase syntactic foams consisting of a close‐packed microballoons bed, binding resin, and air voids are reported. Preparation methods varied from simple dry‐blending with a solid powdery resin‐to‐resin coating of spheres from polymer solutions. By holding the hollow spheres volume fraction constant and at the maximum packing, a property studied was a function of a single variable, namely, resin volume fraction, void content or density. Mechanical properties are reported as function of one of these variables. The temperature effect was investigated on polyimide based foams. The dielectric constant is shown to obey a logarithmic rule of mixtures. Modified modulus‐composition equations are shown to be applicable for prediction of moduli values of the three‐phase systems.