The influence of Korver's de-ashing procedure on the changes in the thermal stability and porosity of three commercial granular activated carbons: D43/1 (Carbo-Tech, Essen, Germany), AHD, and WD-extra (Hajnowka, Poland) was investigated. For original carbons, carbons treated only with concentrated HCl, and completely de-ashed carbons (treated with concentrated HCl and HF) the thermogravimetric results in air and helium, apparent and true densities, low-temperature nitrogen adsorption, and mercury porosimetry are reported and compared. The changes in thermal stability and porosity are discussed together with the mechanism of carbon de-ashing.