A novel, simple electrochemical method is described for measuring salt diffusion coefficients and ion transference numbers in complex electrolyte systems. A theoretical treatment is given for the application of the method to polymer electrolytes in which complex ionic species, such as ion pairs or triplets, coexist with free ions. The applicability of the method to polyether electrolytes in which extensive ion-pairing occurs is experimentally demonstrated. The salt diffusion coefficients and Li+ ion transport numbers in poly(ethylene glycol dimethyl ether) containing LiAsF6, and LiBr are determined.