Syntactic foams, used in submersibles and in pipelines for deep sea oil wells, must be resistant to the severe conditions of the deep sea environment. As these foams will be in service for at least 20 years, their qualification testing is crucial. However, their mechanical characterization under real conditions of use is a challenge. In deep sea, the main loading is hydrostatic compression, however there is no standard procedure for testing material under pure hydrostatic pressure. The aim of this paper is to present a new characterization technique based on buoyancy loss measurement under hydrostatic pressure. To validate the method, two different syntactic foams (one brittle and one ductile) have been tested. Their behaviours under hydrostatic pressure have been followed by the proposed technique. The results from this innovative characterization technique have been compared to those of traditional uniaxial compression tests performed on the same materials.