Ash formation during the bubbling fluidized bed (BFB) combustion of bark and pulp mill sludge has been studied on an industrial and bench scale. During co-firing in an industrial BFB a submicron fly ash mode was formed via condensation of volatilized K, Na, S and Cl species at 0.05-0.3 mu m. The submicron mass mode below 0.3 mu m made up 2.2-5.0% of the fly ash, while the share of the supermicron mass fraction was 93.6-97.2%. Elements depleted in the ultrafine ash were Ca, Si, Al, Mg, Fe, Mn, P and Ti. The bench-scale test showed that the ultrafine particle concentration was increased by a higher bed temperature and decreased due to sludge moisture. As, Cd, Pb and Rb were enriched in the ultrafine ash on a bench scale, while Ba, Co, Sr and V were depleted. Cu and Zn were enriched in the ultrafine ash during the combustion of dried sludge, but not when wet sludge was fired. Micron-size ash particles composed of nonvolatile species, Ca, Si, Mg, Al, P and Mn, adhered to the bed sand, presumably by surface forces, and sintering densified the ash layer.