Surface area, pore volume distribution, and porosity of montmorillonite are determined after being exposed to aqueous solutions with various pH values. For measurement, after each pH treatment the clay samples were freeze-dried in order to keep the structure of the clay same as that in the aqueous solution. Surface area and pore size distribution measurements were performed on an unmodified and four pH (2.5, 4.0, 5.5, 9.0)-adjusted clay samples using N-2 and CO2 as adsorbates. The surface area measurements at lower pH are lower than those measured at higher pH due to the replacement of Na+ ions from interlayers at lower pH which may be followed by the reduction of repulsive forces. As the pH of the montmorillonite increases to 9.0, the surface area increases almost seven times compared to that at pH 2.5. Pore size distribution was determined by CO2 adsorption at 273 K for micropores (<20 Angstrom), by N-2 adsorption at 77 K for mesopores (between 20 and 500 Angstrom), and by using both Hg intrusion and N-2 adsorption for macropores (>500 Angstrom). Since N-2 molecules undergo a molecular sieving at 77 K CO2 gas at 273 K was used to measure the micropore surface area and gave higher surface area measurements than those measured by N-2. Surface area values measured by CO2 adsorption ranged between 30 and 200 m(2)/g for montmorillonite while those measured by N-2 adsorption were 13.7-70 m(2)/g in the pH range studied. (C) 1999 Academic Press.