A bituminous coal with a size faction of 63-90 mu m was combusted under oxidizing atmosphere (air) in a drop tube furnace (DTF) and a pressurized drop tube furnace (PDTF) at a gas temperature of 1573 K and pressures of 0.1, 0.5, 1.0, and 1.5 MPa. Ash generated at high pressure was found to be much finer than ash generated at low pressure due to the differences in char structure. Pressure influences ash formation by its effect on the structure of char particles after devolatilisation. Char samples produced at high pressure have higher proportions of porous char particles which fragment to form finer ash particles. This study confirms and elaborates upon previous studies relating the structure and morphologies of char to ash formation.