The work described in this thesis was carried out as part of a project to explore the use of solid polymer electrolytes as pH electrodes, the properties of which would be superior to those of the fragile glass membranes traditionally used. To this end, potentiometry was used to study ion transport in membranes which were composites of poly(vinyl chloride) and poly(ethylene oxide). The idea was that the hydrophilic polymer (PEO) would interact with water molecules and ionic species, and the hydrophobic polymer (PVC) would serve as an inert matrix, intimately tangled with the hydrophilic polymer to prevent its diffusion into the aqueous solution phase. It was shown that membranes prepared by co-casting polymers from solution were indeed stable in water and that the properties could be changed by change in fabrication procedure which changed the microstructure. This thesis describes the development of the membrane preparation procedure from initially pure PEO gels to processed pure PEO membranes then co-cast composite PEO and PVC membranes and finally rotary evaporated PEO-PVC membranes. The effect of the preparation procedure on the potential difference response of these membranes was investigated to correlate any structural changes to the membrane electrochemical behaviour. Membranes prepared by the various procedures were tested with thermal analysis, FTIR and polarised light optical spectroscopy to observe any trends in physical characteristics associated with structural changes. The electrical properties of these membranes were correlated with the microscopic and macroscopic membrane structure and chemical composition. It is postulated that the membranes are a nano-scale composite structure comprising interpenetrating networks of hydrophobic and hydrophilic regions of size determined by the preparation procedure. The size of these regions determine the transport properties of ions moving through the membrane. It is proposed that the dominant effect on permselectivty in the membranes studied here is a modification of the interaction between ions and water within the membrane channels formed by a hydrated hydrophilic polymer.