Pyrolysis is carried out using beech wood for packed beds, consisting of pellets and particles of various sizes, and a block in the same shape and size as the beds. The different physical properties of the samples affect the heat- and mass-transfer rates and, consequently, the conversion time. However, the effects on the yields and composition of the lumped product classes are generally negligible. An exception is observed for the packed beds made of pulverized material, where re-evaporation and pyrolysis of the vapors, previously migrated and condensed in cold zones, result in reduced yields of some organic products (hydroxyacetaldehyde, levoglucosan, syringols) and an increased carbon content of the char. These effects are enhanced by successively smaller particle sizes and more severe heating conditions.