Surface chemical composition of blend composed of monodisperse polystyrene (hPS) with the number-average molecular weight, M-n, of 19.7k and deuterated monodisperse polystyrene (dPS) with M-n of 847k was analyzed based on time-of-flight secondary ion mass spectroscopy (ToF-SIMS). Although hPS possess higher surface free energy than dPS, ToF-SIMS revealed that hPS was preferentially segregated at the outermost surface of the blend films with various compositions. The surface segregation of hPS can be explained in terms of the molecular weight disparity for both components, i.e., an entropic effect. (C) 2002 Elsevier Science B.V. All rights reserved.