Shock wave equation of state and release behavior of tuff from the Nevada Test Site were measured in the range 8 to 107 GPa by recording shock wave profiles produced by impact of projectiles accelerated by a two-stage light-gas gun. Shock wave profiles were measured with a fast optical interferometer. Hugoniot, sound speed, and effective Gruneisen parameter data were determined from the profiles. The specimen geometry allows direct observation of the shape of the leading edge of the shock, in which complex compressive dynamics caused by pore crushing or phase transitions can manifest themselves. The tuff specimens were tested in both a water-saturated and semidry state. Hydration of the rock increases both the shock and sound speeds for a given compression. The Us-Up relation for our water-saturated tuff is slightly nonlinear and can be approximated as Us=2.00+1.42 Up for 0<Up<6 km/s, and Us=2.29+1.29 Up for 0<Up<3 km/s. For semidry tuff at pressures less than 18 GPa, Us=1.70+1.28 Up. ¿ American Geophysical Union 1994