Adsorption has been investigated using bovine serum albumin (BSA) as the model protein and sulfonated microspheres as the matrix. The adsorption appears to be sensitive to time, pH, and ionic strength, and the presence of sulfonate groups can play important roles in this process, increasing the adsorption rate and the amount of protein adsorbed, as well as influencing the interaction of BSA molecules. Fittings from Langmuir's and Freundlich's isotherms indicate that the actual adsorption becomes more complicated than the ideal process, due to the presence of intermolecular interactions of BSA. The determinations under different pH conditions (pH 2.2, 4.3, 7.4) show that near the isoelectric point of BSA (pI 4.7), the amount of protein adsorbed reached a maximum value, and a higher or lower pH resulted in a significant decrease in amount adsorbed. The effect of ionic strength is, however, closely associated with the operating conditions. (c) 2005 Society of Chemical Industry