This project was directed to the understanding of composites with constituents of negative stiffness. Funds provided by MRSEC were used to refine the method of resonant ultrasound spectroscopy, thereby enabling experimental determination of material properties during a phase transformation, when one phase can be on the cusp of negative stiffness. Preliminary experiments on a metastable alloy were carried out. These experiments were accompanied by a theoretical study of the stability and metastability of systems containing a negative stiffness element. This effort has now been funded by additional external sources and therefore will no longer be supported by MRSEC.