The paper reports the results obtained on the physicochemical characterization of activated composite membranes (ACM). Membrane samples containing different concentrations of di-2-ethyl-hexylphosphoric acid (DEHPA) as a carrier were prepared and in situ characterized by impedance spectroscopy (IS). The results obtained by this technique, based on the linear correlation between the electrical resistance of the membrane and the carrier content allows not only for an in situ characterization of the working membrane but also for a novel analytical method to direct determination of actual carrier content in the membrane. These results also correlate with those obtained by the application of other spectroscopic techniques to ACM samples. Thus, a linear correlation between the carrier content in the membrane and that in the casting solution was obtained from the results of the membrane analysis by using the inductively coupled plasma (ICP) technique. The carrier distribution in the membrane was evaluated by X-ray microanalysis (EDS), which allowed us to determine that DEHPA is trapped in the polymeric phase of the composite membrane. The results obtained by the indicated techniques correlate with good agreement and allow characterization of the membrane performance under different working conditions.