The electrical double layer (EDL) regions in ionic liquid (IL) solvents demonstrate behaviors that are not easily explained by a priori knowledge of aqueous counterparts. Despite previous and ongoing research efforts, debate surrounds the causes and prediction of these behaviors. A possible pitfall is the range of ionicity in IL cation/anion pairs. One relatively new and powerful tool useful for exploring the solid-liquid interface that can address these questions is large-amplitude Fourier transform alternating current (FT-ac) voltammetry. By measuring the FT-ac voltammetry signal in potential windows containing only double-layer charging current, the characteristic morphology of ILs’ EDLs can be revealed. This work will discuss the interfacial capacitance of protic and aprotic ILs systems and develop a relationship to fundamental harmonic components of the FT-ac data across metallic and non-metallic working electrodes. The experimental data will be compared with existing theoretical models to advance a working picture of the IL EDL within the capacitive current regime.