The intercalation process of acid-treated montmorillonite clays by a cationinc surfactant (decyltrimethylammonuium) from a hydroxide solution was affected by the temperature of acid activation. Although the cation exchange capacity of the treated clay at 90 degrees C (0.74 mequiv g(-1)) was lower compared to that treated at room temperature (0.84 mequiv g(-1)), the uptaken amount of the surfactant (1.24 mmol g(-1)) and thus the basal spacing (3.83 nm) were higher. These values depended on the initial loading concentrations. However, when the clay was treated at room temperature, the uptaken amounts of surfactant (0.81 mmol g(-1)) and the basal spacing (2.20 nm) were lower. These values were independent of the initial loading concentration. The higher basal spacing (3.83 nm) was also affected by the type of the exchange medium and the washing solution by a mixture of ethanolic solutions. The intercalation of the surfactants occurred in two different ways, and was related to exfoliation properties of the acid activated clays. The intercalated surfactant exhibited different conformations in the interlayer space and different thermal stability.