A kinetic model was developed to characterise the heterogeneous nature of kraft delignification kinetics, taking into account the effect of chip thickness. The final form of the model applied to kraft delignification can be represented by a first-order rate equation with a rate constant inversely proportional to a power function of chip thickness. Published laboratory results from kraft pulping of various lignocellulosic materials were used to validate the model. The outcomes confirm the significant effect of chip thickness on the delignification rate. The resultant model was used to predict the effect of chip thickness on the kappa distribution of kraft pulps.