This paper reports quantitative characterization of ash deposits from a 330 MWe power station boiler. The deposit samples were cut into a series of thin slices, and a systematic characterization was carried out on these slices to understand deposit structural, chemical, and mineralogical heterogeneity corresponding to different stages of deposit formation and growth. It is found that the deposits consist of distinctive layers of different textures, with the inner regions appearing to be more compact. Quantitative analyses of deposit cross-sectional SEM images showed an increase in both the porosity and the size of the sintered ash particles with deposit growth. For the deposit above the burner, the porosity was found to increase from approximately 60−70% at the contact surface to approximately 80% at the outer layer, while for the deposit below the burner, there is an increase of porosity from approximately 10−20% at the contact surface to approximately 40−60% on the outer layer. The chemistry and mineralogy of the different deposit slices are similar, indicating that deposit heterogeneity is dominantly present in the physical structure. Thermodynamic calculations, which were carried out based on the chemical compositions of each deposit slice, failed to predict the mineralogy of the deposit slices indicating that ash deposition is a nonequilibrium process and the deposit growth is highly dynamic.