The adsorption of acrylic acid and maleic acid on alumina was studied, and it has been found that the adsorption obeys the Langmuir isotherm. The adsorption of the acids is dependent on the pH of the solutions. For acrylic acid, the optimal pH is 4.5; for maleic acid, the optimal pH is 1.1-1.6. These pH values are close to the pK(alpha) for the acrylic acid and pK(alpha 1) for maleic acid, respectively. The maximum amount of each acid adsorbed at its optimal pH was obtained by fitting the adsorption data to the Langmuir isotherm, and the values are 727 mu mol/g for the acrylic acid and 525 mu mol/g for maleic acid. These values are consistent with monolayer adsorption. Solid state C-13 NMR was used to investigate the nature of the adsorption. It is suggested that the adsorbed carboxylic acids are present in the undissociated form.