Sepiolite, acid treated under different molarity of HCl at various times, has been studied to find the best acid treatment conditions for obtaining anhydrous sepiolite without structural folding and with fully open and empty channels. High molarity (e.g., 3 M) acid treatment causes severe acid attack and heterogeneous magnesium dissolution from sepiolite particles while the low molarity (e.g., 0.5 M) acid treatment dissolves magnesium homogeneously thorough the particles. This, in turn, affects the behaviour of sepiolite upon calcination at 450ºC in that homogeneous magnesium dissolution gives better structural stability and thus higher amount of open channels with the least amount of magnesium removal from the sepiolite structure. This is verified by specific surface area measurements and rehydration behaviour after the calcination. No change in the morphology of sepiolite fibers was observed after acid treatment.