Sound speeds, at pressure, and the overdriven Hugoniot were measured for the plastic‐bonded explosive PBX‐9501. The two curves intersect at the Chapman–Jouguet (CJ) state because of the sonic conditionD=c+u. This permitted a novel determination of the ‘‘thermodynamic’’ CJ pressure. A value of 34.8±0.3 GPa was obtained. The data permit a direct experimental determination of the isentropic gamma, &ggr;S=−(∂lnP/∂lnV)S, and the Gru¨neisen parameter, &ggr;=V(∂P/∂E)V, in the overdriven pressure range. ©1996 American Institute of Physics.