The kraft process will certainly play a crucial role in many future biorefineries. It is a globally relevant, large-scale process based on mature technology, which can already today produce several valuable side streams, e.g. lignin and tall oil. Nevertheless, further improvements to the process, such as to obtain new side streams and enhance material and energy efficiencies, necessitate a thorough understanding of its underlying mechanisms. Under the historical assumption of a reaction-controlled process, lignin inter-unit linkages have been the main focus of past research, whereas only more recently have other mechanisms gained more attention. Consequently, a mechanistic understanding of the kraft process is lacking in several aspects, including the impact of the heterogeneous nature of lignin and its effect on the delignification kinetics.Our work has expanded on this topic by comparing the relative kinetics of reaction and mass transfer mechanisms during kraft pulping of birch wood meal. A novel model has been developed, incorporating both reaction chemistry and diffusion by relating the diffusivity of dissolved lignin fragments to their molecular weight. Model parameters were supported by experimental data from simulated kraft cooks at varying process conditions using a small-scale flow-through reactor.The results showed that kraft delignification could, at the cell wall level, be accurately represented by a diffusion-controlled model. The present model employed simpler reaction expressions than traditional models, while still predicting the delignification behaviour throughout the entire process. Moreover, the model could also reasonably predict the development over time of the molecular weight distribution of dissolved lignin. In addition, the results showed a previously unreported variation in the molecular weight distribution of lignin in the presence of sulphide ions, which in turn could be related to the interplay between reaction and mass transport kinetics.