Tighter control of expanded bed adsorption (EBA) separations is shown to be possible through the combined use of on-line monitoring of samples abstracted from within a column and an adsorption model. A 5 cm diameter EBA column was modified to allow liquid sampling from within the fluidized bed during the adsorption of pure lysozyme and bovine serum albumin (BSA) on STREAMLINE SP and STREAMLINE DEAE, respectively. The developing breakthrough curve was measured at various positions along the bed. The adsorption model incorporated terms for liquid dispersion, film mass transfer, pore diffusion and adsorption. Parameters obtained by fitting the model to the experimental in-bed breakthrough curves were subsequently used to predict breakthrough at the bed exit. Comparison of the actual breakthrough curves at the bed exit (40 cm sample port) with those predicted based on the fit of the model to the experimental breakthrough data obtained from the 25 cm sample port, demonstrated that on-line, in-bed monitoring of the progress of the front of non-bound protein can provide information for tighter control of EBA separations. The accuracy of the model predictions was improved by employing information on the axial variations in the bed voidage, liquid phase axial dispersion and dynamic capacity for the top and bottom zones within the column. Ten per cent breakthrough at the 40 cm sample port was predicted to within 2% and 5% of the breakthrough time observed for the experimental runs for lysozyme and BSA, respectively.