To better understand the mechanisms of nanofiltration (NF) and to be able to perform its predictive modelling one needs to know electrochemical properties of membrane active layers. Their determination is complicated by the multilayer structure of NF membranes. Conventional linear steady-state techniques yield information on the membrane average properties (involving both active layer and support) alone. In this paper we develop a theory of non-steady-state membrane potential and analyse the informativeness of various modes of its experimental measurement. In particular, we come to the conclusion that any solution replacement technique is not capable of yielding information on the so-called initial membrane potential in membranes with thin and not too dense active layers due to the presence of unstirred solution layers near the membrane surface. A novel non-steady-state technique ('dab' technique) is put forward for the measurements of transient membrane potential. A membrane preliminary equilibrated with an electrolyte solution is touched by a drop of solution of different concentration, Thus, a concentration gradient initially located within a very thin surface layer is created. An electrical response to this is tracked with a pair of reversible electrodes, That makes possible the determination of ion transport numbers within active layers. The potentialities of the new technique are illustrated by sample measurements performed with a composite NF membranes (PES10) and two monolayer membranes (Cellophane and Nafion(R)115). The PES10 membrane is shown to have quite a high electrochemical activity. The Nafion(R)115 membrane is shown to be macroscopically homogeneous.