During the investigation into the possibility of purifying an API intermediate from a dimeric impurity via organic solvent nanofiltration (OSN). A change in rejection profile was noted on the addition of a strong acid to a solution of the amine in the mixed solvent system of tetrahydrofuran-water (80:20). This change in rejection did not occur with the impurity, thus making the separation via OSN far more effective. The requirement of all three components i.e. acid, organic solvent and water to achieve this change in rejection was also demonstrated. Additionally the type of acid, quantity and the effects of the acidic solution on membrane stability were also examined. (C) 2013 Elsevier B.V. All rights reserved.