In this study, dielectric properties of sulfonated poly(ether ether ketone) (SPEEK) solid polymer electrolytes (SPEs) containing room-temperature molten salt, 1-ethyl-3-methylimidazolium tetrafluoroborate were investigated. Additionally, the effects of ionic liquid (IL) addition on the thermomechanical properties and the microstructure of composite electrolytes (SPEEK-30 x 1IM and SPEEK-30 x 2IM) were studied. The characteristics of the SPEs were investigated by FTIR, SEM, TGA, DSC and DMA measurements. FTIR analysis confirmed the success of the sulfonation process and IL/sulfonated polymer interactions. IL addition caused changes in the crystalline structure and surface morphology of the SPEs. It was observed that SPEEK-30 x 1IM and SPEEK-30 x 2IM electrolytes have lower glass transition temperatures (T-g) and crystallinity than that of pure SPEEK-30. Due to the amorphous character formed by IL, proton conduction paths in which ILs exhibit enhanced conductivity have been formed. Among the composite electrolytes, SPEEK-30 x 2IM demonstrated an ionic conductivity (5.45 x 10(-4) Scm(-1)) more than 20 times higher than SPEEK-30.