Abstract Heterogeneous catalysts, especially those produced with functionalized clays, can be used for methyl transesterification at moderate temperatures. The use of bentonite clay functionalized by potassium compounds (KF, KOH, K2CO3, KCl and K2SO4) in the methyl transesterification of cottonseed oil is the object of the present study. The catalysts were produced by contacting aqueous solutions with the clay and subsequent drying. The catalysts were characterized by Hammett indicators to determine alkalinity. The methyl transesterification reactions were made in a flask with stirring and reflux at 70°C for 2 h. The performance evaluation of the reaction was carried out using regression between the absorption band area and the concentration of a biodiesel-oil mixture by infrared spectroscopy. The catalytic activity is directly associated with the alkaline character of the catalysts. The catalysts produced with KCl and K2SO4 did not show conversion in the methyl transesterification reaction, while the catalysts produced with KF, KOH and K2CO3 showed a conversion greater than 90%.