The electrolytic behaviour of ethylmagnesium bromide in poly(ethylene oxide) was investigated. Besides polymeric ether, the electrolyte solution contained a small amount of monomeric ether (THF or DEE). Best conductivities were obtained with an ethylene oxide to Mg ratio of 4. For the electrolyte system EtMgBr-P(EO)(4)-DEE, an electrical conductivity of 0.1 mS cm(-1) at 40 degreesC was found. Using these electrolytes, magnesium can be deposited by cathodic reduction as well as dissolved by anodic oxidation. For a magnesium electrode in contact with these electrolytes, an OCV of - 1.9 V (vs. Ag-AgCl) was measured. The kind of monomer ether influences significantly the structure of the electrolyte systems. DSC and XRD measurements of the THF-containing, electrolytes showed a decrease in content of the crystalline PEO phase with increasing content of EtMgBr, which is not the case for the DEE-containing systems.