Dissolution rates of several clay materials (reference kaolinite samples, natural kaolinites of different origin, illitic clays, halloysite) in 0.5 and 5 M sulphuric and 1 M hydrochloric acids were determined by measuring the release rate of aluminium into solution. The X-ray powder diffraction and X-ray fluorescence methods have been employed for mineralogical and chemical analyses of clay samples, respectively. The surface area of the samples was determined by a N₂ adsorption technique (BET). The dissolved Al concentration was determined by spectrophotometry. The dissolution rate of kaolinite in 0.5 M sulphuric acid at 25 °C is approximately three times higher than in hydrochloric acid of formally equivalent H⁺ concentration. The dissolution in 5 M sulphuric acid is eight times faster if the solid phase is periodically separated from the acid solution, washed by distilled water and dried. The aluminium release rate decreases with the increasing amounts of clay micas in kaolinitic clays and is affected by the concentration of Al in the mineral structure. Crystallinity, as a factor reflecting the quality of the crystal structure, influences significantly the aluminium release rates during dissolution by acids.