An unsteady-state mathematical model describing supercritical fluid (SCF) delignification of wood in a packed bed was developed. Equations describing intraparticle supercritical methylamine reaction with lignin, and subsequent transport of the methylamine-lignin complex into the bulk phase were derived and solved using orthogonal collocation. Model results for the case of supercritical methylamine are presented and compared with experimental results. Simulation results correctly predicted the experimental results. Sensitivity of methylamine extraction of lignin was evaluated by varying bulk fluid velocity, mass transfer coefficient, and particle diameter. Delignification was a strong function of particle diameter and a relatively weaker function of fluid velocity and mass transfer coefficient. The Thiele modulus and Biot number showed that delignification was reaction rate limited for wood particle diameter values less than or equal to 0.1 cm, but delignification was limited by intra-particle mass transfer for wood particle diameter values greater than or equal to 1.0 cm.