We present new data on the unreacted approximate isentrope of the HMX‐based explosive LX‐04, measured to 170 kbar, using newly developed long pulse isentropic compression techniques at the Sandia National Laboratories Z Machine facility. This study extends in pressure by 70&percent; the previous state of the art on unreacted LX‐04 using this technique. This isentrope will give the unreacted Hugoniot from thermodynamic relations using a Gruneisen equation of state model. The unreacted Hugoniot of LX‐04 is important in understanding the structure of the reaction front in the detonating explosive. We find that a Hugoniot given byUS= 2.95 km/s + 1.69uPyields for an isentrope a curve which fits our LX‐04 ICE data well. © 2004 American Institute of Physics