Reformulation of the standard equations governing internal reaction of carbon, used to predict the Zone I−Zone II transition temperature, T c, shows that T c is, primarily, an inverse function of particle radius (a). The expression obtained has the form:  log(a) ∝ (1/T c). This result is supported by the (limited) experimental data available. The numerical values also show that, for particles of pulverized coal size, the values of T c are in the flame temperature range, thus possibly allowing a discontinuous drop in reaction rate toward the end of a flame; this has relevance to increased LOI due to staging for NOX control. The results also permit evaluation of the internal diffusion coefficient indicating that internal reaction most probably involves only the macropores, and total internal surface area measurements are probably irrelevant in determining reaction rates.