The advantages and disadvantages of the PVC matrix of ion-selective electrodes are briefly discussed. Some of newly designed ion-selective electrodes (ISEs) with PVC membranes are mentioned. Emphasis is laid on what seems to be known about PVC membranes and on some of the problems which remain unsolved, as yet. The selectivity patterns of neutral carrier-based membranes and the so-called anion interference with the cationic response of the membranes are discussed in the frame of a thermodynamical approach. It is shown that this approach, especially taking into account the possibility for any ion to present in a membrane a number of different species ('multi-species approach') allows to describe in detail the selectivity as well as the anion interference. Among the problems which remain unsolved, three questions are discussed: (i) whether the diffusion potential contributes significantly to the membrane potential; (ii) what is the origin of the cation permselectivity of neutral carrier-based membranes, and (iii) what is the role of the PVC in membranes beside its mechanical role.