Adsorption behavior of daidzin, genistin, 6 ''-0-malonyldaidzin and 6 ''-0-malonylgenistin, the four major soy isoflavones presented in soy molasses centrifugation supernatant on activated carbon was studied in this paper so as to provide theoretical basis for the purification of soy oligosaccharides from soy molasses. Kinetic experiments showed that the adsorption processes obeyed pseudo-second-order kinetics and equilibrium was nearly achieved in 90 min. Weber-Morris model fitting showed that adsorption process consisted of 3 stages: boundary layer diffusion and two intra-particle diffusions. Experimental adsorption data for every isoflavone components could be described separately by the Langmuir isotherm model and the calculated maximum adsorptions were in the order of genistin > daidzin > 6 ''-0-malonylgenistin > 6 ''-0-malonyldaidzin, indicating that the adsorption driving forces were due to dispersion interactions between the aromatic ring of isoflavone and the aromatic structure of the activated carbon. Adsorption behaviors of isoflavones on activated carbon in sugar free solutions were compared. It was found that, by removing sugar from the system, diffusion rate constants and the sum of the maximum adsorption capacity increased. (C) 2012 Elsevier B.V. All rights reserved.