We report on three different in situ and ex situ concentration measurement methods in symmetric lithium/polymer-electrolyte/lithium cells. We examine our results on the basis of a simple calculation of ionic concentration within the electrolyte; in the case where no dendrite is observed, this calculation accounts quantitatively for all experimental results. In the case of dendritic growth, we can measure the concentration distribution around the dendrites; this permits correlation of the active parts of the electrodes and of the growing dendrites with local ionic depletion in the vicinity of these active parts.