A gel electrolyte was produced by photo-polymerisation of poly(ethyleneglycol diacrylate) in an electrolyte mixture containing LiPF6 in propylene carbonate in a cell containing lithium metal electrodes. The cell was placed in a Schlieren optical set-up and a potential difference was applied across the electrodes. Using the optical set-up, the change in refractive index across the cell could be measured and then translated into concentration values. In this way, concentration change across the electrolyte could for the first time be measured in situ while monitoring the current passing through the cell during the application of an external potential difference. After the applied potential difference had been removed, the decay of the concentration gradient across the cell and the open circuit potential were monitored. Good agreement between the experimental results and the theory was observed. The transport number estimated for lithium using the concentration difference values was in good agreement with the value estimated on the basis of the diffusion coefficients of the anion and the cation. (C) 2000 Elsevier Science B.V. All rights reserved.