A treelike model has been proposed for the structure of lignin. Kinetics of delignification are formulated to account for the cleavage of linkages along the linear primary chains and at the cross-links. Experimental data obtained from isothermal delignification are used to verify this theory. Good agreement has been obtained in calculating the delignification curves. Activation energies are found to be 172 kilojoules per mole of cross-links and 132 kilojoules per mole of aryl ether bonds.