Differential anomalous X-ray scattering (DAS) and extended X-ray absorption fine structure (EXAFS) were used to determine the local structure surrounding the ions in amorphous polymer electrolyte complexes of poly(propylene oxide) and ZnBr2 : [PPO]6ZnBr2 and [PPO]16ZnBr2. On the basis of our results, we propose that ZnBr2 dissociates to form mainly ZnBr42- and Zn(O)(4)(2)+, where (O) is an ether oxygen on a PPO chain. This observation is contrasted with aqueous ZnBr2, where ZnBr2, ZnBr3-, and ZnBr42- are all observed in the estimated proportions of 2:4:3, respectively. This paper describes the DAS method and its application to structure determination in a multicomponent, amorphous system. In systems with n components, where one or more of the species is a heavy atom, DAS reduces the scattering intensity from a weighted sum of n(n + 1)/2 partial structure factors to a sum of partial structure factors which only include the heavy atom of interest.