A mathematical model for the swelling of a coal and the formation of a char structure during devolatilization is developed, based on a simplified multibubble mechanism. The formation of a char has been considered as having two successive steps: a multibubble stage, followed by a single-bubble stage. During the multibubble stage, the rupture of bubbles is governed by a force balance and the release of volatile matter is determined by the rate at which bubbles rupture. When a cenospheric structure has formed, the single-bubble model applies. During this stage, the rupture of a bubble is determined by its wall stress, and volatile species are released through both bubbles rupturing and direct diffusion of volatile species to the particle's surface. A sensitivity study has been carried out, based on which parameters have been determined for the present modeling work. The model predicts experimental trends for the swelling of a coal and the structural characteristics of chars under different heating conditions. The model is the first to provide a complete quantitative description of the evolution of a char's structure for a pulverized coal during rapid heating. Using standard properties of density fractions of a coal, the model predicts the heterogeneity of a char's structure and quantifies the distribution of different structures. The results predicted by the model agree with experimental measurements. (C) 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved.