The membrane aging phenomenon and its influences on the kinetics of cobalt(II) permeation by the liquid surfactant membranes (LSM) process has been studied. The experimental results showed that the emulsion formulated from the internal aqueous phase and the membrane (organic phase) of a certain age has far-reaching consequences on the kinetics of metal extraction and emulsion stability. The investigations have revealed that there exists an optimal age and composition of the membrane in which the highest extraction (synergism) and the lowest emulsion swelling are achieved. Analytical and instrumental examinations of the membrane phase during aging have shown various chemical and physical changes as a result of the surfactant decomposition, reaction products precipitation and self-association, and macroemulsion formation.