This study concentrates on time-optimal operation of a general batch diafiltration process. The process model consists of a set of input affine differential equations. We apply Pontryagin's minimum principle to formulate necessary conditions of optimality. These are then used to identify the shape of optimal control as well as singular surface in concentration space. Comparison is made between traditional control techniques and obtained minimum time control. It is shown that traditionally used operations are suboptimal for selected case studies. © 2012 IFAC.