Experiments have been done to obtain product isentropes of the plastic bonded TATB explosive PBX 9502. A thin aluminum flyer was thrown at an explosive disk so as to generate a state in excess of the Chapman-Jouguet pressure. Because the flyer is thin, the shock wave is followed by a release wave that lowers the pressure in the explosive products. The explosive disk was backed by a LiF tamper/window. Particle velocity at the explosive—window interface was measured with a velocity interferometer (VISAR) and measured particle velocity records were used to construct isentropes. Unexpected behavior is seen in the shape of the isentropes. Experimental details and the resulting isentropes will be presented. ©2000 American Institute of Physics.