Bentonite occurs in the Janggi Conglomerate of Tertiary age and consists mainly of montmorillonite with Mg as predominant interlayer cations. The bentonite was reacted with various concentrations of sulfuric acid (0.8~1.5M) for various reaction time (1-10h) at $103^{\circ}C$. Cation exchange capacity, exchangeable cations, surface area and solid acidity of the original bulk and acid activated bentonites were measured. Chemical analysis, X-ray diffraction, differential thermal analysis and infrared spectroscopy were used to characterize the changes in structure and properties of the acid activated bentonite. The dissolution of octahedral cations occurs not only from the edge of the clay platelets but also throughout the whole clay structure creating vacant octahedral sites. These lattice defects are created by $H^+$ diffused into the smectite layers. The cations leached possibly from the octahedral sheets are adsorbed on the interlayer exchange sites. They are exchanged with hydronium ions again by stronger acid attack. These reactions create wedge-shaped pores resulting in the increase of the surface area and the changes the morphology in the lattice structure.