Two montmorillonites STx-1 (Texas) and SWy-2 (Wyoming) were first activated with different amounts of 12 M HCl and then exchanged with a fixed amount of 1 M tetramethylammonium (TMA(+)) chloride solution (H/TMA samples) at room temperature. TMA(+)-exchanged samples and then acid activated (TMA/H samples) were also prepared to evaluate the resistance to displacement of TMA(+) by protons. The surface area and the acidity were determined by BET and cyclohexylamine adsorption methods, respectively. The catalytic activity of these acid activated organoclays was directly measured using the isomerization of I-butene at 300 degreesC to yield cis- and trans-2-butene. The total conversion for the isomerization of I-butene was higher for the TMA/H-samples (70 and 47% for SWy-2 and STx-1, respectively) than for the H/TMA-samples (51 and 25% for SWy-2 and STx-1, respectively). TMA(+) cations adsorbed on the clays were extremely resistant to exchange by protons, but protons were easily displaced by TMA(+) cations.