The leaching of ash in sulfuric acid (pH 1.0, 25EC, S/L 1:10) was studied as a function of time. Data for the acid consumption and the extraction of dominant ions are presented. The study embraced 14 coal- ashes, including lignite-ash and ash from the combustion of asphaltites and softwood sawmill residues. The kinetics of acid consumption for ash treatment was compared with the acid consumption of the corresponding ground coals (250-1000 _m fraction). The whole coals consumed acid rapidly during the first few minutes, followed by almost negligible consumption. Coal-ashes consumed acid in two stages, one rapid extending < 30 min and one slow extending ca. 10 days. The rapid phase was dominated by the dissolution of Ca, K and Mg ions for wood-ash, by Ca, Al and Mg ions for lignite-ash and Ca and Mg ions for ash from asphaltites. The slow phase was different for the different fuels, but embraced the leaching of several elements - Fe, Al, K, Na and Mn - with kinetics well described by a diffusion controlled shrinking core model (SCM). The sulfur concentration in solution and the concentration of Ca, Fe, K, Mg, Na, P, Al and Mn in the aqueous phase verified the neutralizing capacity of the untreated ashes as well as the formation of insoluble sulfates in the residues.