The adsorption of chlorophyll-a on bentonite desiccated at 110 degrees C, untreated and acid-treated with H2SO4 solutions over a concentration range between 0.25 and 2.50 mol dm(-3), from acetone solution at 25 degrees C has been studied. The adsorption isotherms may be classified as using Giles’ classification, as type S (untreated sample and 0.25 mol dm(-3) H2SO4-treated sample), type H (0.50 mol dm(-3) H2SO4-treated sample) and type L (1.00 and 2.50 mol dm(-3) H2SO4-treated samples). This fact suggests that the bentonite surfaces (low, high and medium affinity, respectively) behave in differently relation to the adsorption of the chlorophyll-a molecules. The experimental data points have been fitted to the Freundlich equation in order to calculate the adsorption capacities (K-f) of the samples; K-f values range from 0.43 mg kg(-1) for the untreated bentonite up to 108.89 mg kg(-1) for the 0.50 M H2SO4-treated bentonite. The removal efficiencies (R) have also been calculated and range from 5.71% for the untreated bentonite up to 85.18%, for the 0.50 M H2SO4-treated bentonite.