For the characterization and development of new electrolytes it is important to know the number of free charge carriers and their mobility. A new method has been developed to estimate the charge carrier density, n, and mobility, mu, for an electrolyte using dielectric measurements and a space charge relaxation model. In order to verify the applicability of this method, it has been tested with an aqueous KI solution electrolyte. The method was then applied to a polyethylene oxide (PEO) based polymer electrolyte. In the case of the polymer electrolyte, plasticized and non plasticized electrolytes intended for photo-electrochemical solar cells containing Pr4N+I- salt which is an iodide ion conductor at room temperature was used. The significant conductivity increase that occurs during the PEO crystallite melting is shown to be due to a large and abrupt increase of the concentration of mobile charge carriers. It is also shown that, for the PEO above the crystallite melting temperature, the introduction of the plasticizer results in an increased mobility. Results from the solution electrolyte show good agreement with known values. (C) 2011 Elsevier B.V. All rights reserved.