Exothermic chemical reactions are capable of autocatalytic self-propagation in a condensed reactive medium as a thermal wave. These take place in self-propagating high-temperature synthesis, frontal polymerization, curing of epoxy resins, and various solid-phase reaction processes. The lack of accurate analytical solutions for the front velocity and spatial parameter distributions has led to a period of approximate analytical solutions based on the infinitely narrow reaction zone assumption. A more accurate reaction zone approximation is presented that results in new equations for temperature and concentration spatial distributions as well as the wave velocity. Necessary conditions for nondegeneration of the combustion wave were also obtained.