Equilibrium adsorption experiments were carried out to investigate the effect of pH and ionic strength on competitive adsorption of erythromycin A (EA) and C (EC) in aqueous solution onto macroporous resin, Sepabead SP825, at different temperatures. The equilibrium data for the binary system were analyzed using the extended Langmuir isotherm, and the characteristic parameters were determined. The maximum adsorption capacities for both EA and EC were obtained at pH 8.5, ionic strength 0.4mol center dot L-1 NaCl, and 303K. Selectivity of Sepabead SP825 for EA and EC was also studied. The results suggested that EA was preferred over EC, and the increase of ionic strength and temperature in neutral solution was to the benefit of the separation of EA and EC.