The pinoresinol unit is one of the principal interunit linkages in lignin. As such, its chemistry and properties are of major importance in understanding the behavior or the polymer. This work examines the homolytic cleavage of the pinoresinol system, representing the initial step in thermal degradation. The bond dissociation enthalpy of this reaction has been evaluated using M06-2X density functional calculations. Products that allow for extensive electron delocalization are energetically favored. Calculations on subsequent reactions reveal a preference of intermediates with two unpaired electrons over a proposed four unpaired electron structure.