Mixture theories are used to model reacting porous solids such as granular explosives. Volume fraction variables are incorporated into these theories because the solid and gaseous reaction products occupy physically distinct volumes within the mixture. Volume fraction is often treated as a kinetic state variable. However, a simple decomposition of the motion demonstrates that it is not a kinematical variable. Indeed, this decomposition yields another variable, the distention, which is kinematical. By using distention as a state variable we suggest a fundamental and important alteration of the constitutive description of reacting porous solids. ©2000 American Institute of Physics.