A new experimental system has been developed, which enables measurements of linear as well as non-linear complex conductivities to be made. The frequency dependence of linear to fifth-order non-linear complex conductivities at different temperatures in a polyethylene oxide/salt complex can then be measured. A characteristic conduction relaxation phenomenon was observed in the spectra, which suggested the existence of different ion-conduction mechanisms between the high- and low-frequency regions. It was also found that the ratio of linear to non-linear conductivities was closely related to the elementary process of ionic transport. Furthermore, this ratio obtained from non-linear measurements allowed an estimate of the important parameters which characterized ionic transport in ion-conducting polymers, such as the hopping distance of an ion or the size of a connected cluster of the site capable of ion hopping, without the need for any additional assumptions. Thus, it was found that in a polyethylene oxide/salt complex, the typical size of a connected cluster of the effective sites capable of ion hopping was approximately 4 nm.