Commercially available solvent stable polymeric nanofiltration membranes were used to study the nanofiltration and diafiltration operations in the context of the pharmaceutical industry. Experimental results are presented for a two-step operation involving the preconcentration of a feed solution via nanofiltration followed by the replacement of the first stage solvent with a second solvent via diafiltration in two stages. Membranes MPF-50 and MPF-60, having molecular weight cut-offs of 700 and 400 respectively, were used in the present study. A solution of erythromycin (MW 734) in ethyl acetate was preconcentrated via nanofiltration followed by replacement of ethyl acetate with methanol via batch diafiltration. The experiments were carried out at 440 psig (3033.8 kPa) and room temperature. Membrane compaction, during the initial period of each operation, affected the solute rejection and permeate flux. High erythromycin rejection (96 % ±) was achieved with the MPF-60 membrane. During the diafiltration operations the membranes exhibited no selectivity for the solvent mixture, irrespective of the feed concentration. Ethyl acetate concentration was reduced to less than 4 % over two batch diafiltration runs.