Sequential adsorption of carboxylated pullulan and pullulan on polystyrene-coated wafers was studied by reflectometry. The incoming pullulan partially displaced the preadsorbed carboxylated pullulan from the surface. Mixed adsorption layers were formed. By increasing the pH of the solution the carboxylated pullulan dissociated and desorbed, whereas the neutral pullulan was not influenced and remained adsorbed. A positively charged polyelectrolyte (quaternized poly(vinylpyridine)) was used as a probe for detecting the amount of trapped, negatively charged carboxylated pullulan remaining on the surface after the desorption step. This trapped carboxylated pullulan is retained by pure entanglement effects. By a mass balance calculation, the fractions of displaced, desorbed, and trapped carboxylated pullulan could be found for pullulans with different molecular weights. The shorter pullulans did not displace any carboxylated pullulan but trapped about half of the layer; the other half could desorb rapidly upon pH increase. Longer pullulan molecules could displace up to 70% of the carboxylated pullulan layer, so that smaller quantities remained to be trapped. Upon pH increase, another 10-15% desorbed, about as much as the amount trapped. Hence, during sequential adsorption both rapid displacement and trapping occur but to different extents depending on the molar mass of the polymer supplied last.