The pyrolytic behavior of a cylindrical packed bed of beech wood pellets, uniformly heated along the lateral surface, is studied. As the heating conditions are varied, the trends shown by the mass loss rate and the yields of the lumped product classes are in agreement with previous literature. Moreover, detailed measurements of the thermal field permit, for the first time, the observation at the central core of the bed, of sequential (a) exothermic, (b) endothermic, and (c) exothermic effects. The examination of the corresponding temperature ranges indicates that they are linked, in the order, to the (primary and secondary) degradation of (a) hemicellulose, (b) cellulose and lignin, and (c) lignin. The dependence on the heating conditions is also explained of the center temperature overshoot (with respect to the steady value) which attains maximum values around 90 K.