Critical deposit properties such as thermal conductivity and strength are thought to be strongly dependent on deposit microstructure. Image analysis techniques were applied to scanning electron microscopy images of ash deposit cross sections to measure quantitatively the microstructure of two ash deposits formed while firing sub-bituminous coal in a pilot-scale combustor under different temperature conditions. The measurements indicate that the high-temperature deposit has a more interconnected and coarser structure than the low-temperature deposit. Little spatial variability of the structural parameters was observed, except for a significant gradient in the contiguity (a measure of particle interconnectedness) across the high-temperature deposit. The measurements are compared to predictions of a ballistic deposition model. Both sampled and simulated deposits are coincident in terms of the measured structural parameters. The results suggest that temperature and composition can be used to predict the effects of coal quality and combustion conditions on the initial microstructure of an ash deposit. Higher-temperature conditions create a deposit with a more porous structure but whose particles are more interconnected because of deformation.