The effects of high-temperature (90 degrees C) sulphuric acid dissolution on the physical and chemical properties of a commercial vermiculite sample, in natural and heated forms, were investigated using chemical, X-ray diffraction, Fourier transform infrared spectroscopy and thermal analyses, and specific surface area measurements. The X-ray diffraction analyses showed that vermiculite and mixed-layer micavermiculite structures in the natural sample were completely destroyed following dissolution in 4 N H2SO4, whereas the basal peak related to mica, which exist in minor amounts in the natural sample, was observed in the X-ray diffraction patterns even after the dissolution in 8 N H2SO4. As a result of the destruction of main clay structures in both samples by acid dissolution, hydrous amorphous silica phases were formed. The maximum specific surface area values were obtained after dissolution in 4 N H2SO4 solution, where the increases in surface area values were approximately 150 (from 3.3 up to 494.6 m(2)/g) and 24 (from 14.0 up to 335.2 m(2)/g) times in the natural and heated samples, respectively. The results of all experiments indicated that the heated sample had higher resistance to acid dissolution than the natural sample, probably due to its dehydrated and collapsed mica-like layer content, which was formed by the heating process at 900 degrees C applied before the dissolution experiments. Besides, in this study, the changes in some of the magnetic properties of the heated sample, before and after dissolution, were also determined.