An innovative processing of syntactic foams based on high-glass transition temperature thermoplastic is reported. The aim is to propose an insulating material able to withstand both continuous effluent temperature up to 150 degreesC and hydrostatic pressure up to 300 bar. Uniaxial compression and tension tests have been performed. Two factors seems to govern the mechanical properties: the wall thickness/radius ratio (e/r) and the volume fraction of microspheres. At room temperature, the study of the strain recovery at the yield stress shows that the plastic deformation is negligible compared to the elastic and anelastic part. The study of the yielding behavior at room temperature has shown a large influence of the type of microsphere compared to the influence of the volume fraction. The syntactic foams exhibit good performance at 150 degreesC as expected from the matrix selection. Comparison with a few classical models have been done. (C) 2003 Kluwer Academic Publishers.