We have measured the third-order permittivity spectra ε3 3 of a monocationic and of a dicationic liquid close to the glass transition temperature by applying ac electric fields with large amplitudes up to 180 kV/cm. A peak (“hump”) in the modulus of ε3 3 is observed for a mono-cationic liquid after subtraction of the dc contribution from the imaginary part of ε3 3. We show that the origin of this experimental “hump” is a peak in the imaginary part of ε3 3, with the peak height strongly increasing with decreasing temperature. Overall, the spectral shape of the third-order permittivity of both ionic liquids is similar to the predictions of a symmetric double well potential model, although this model does not predict a “hump” in the modulus. In contrast, an asymmetric double well potential model predicts a “hump,” but the spectral shape of both the real and imaginary part of ε3 3 deviates significantly from the experimental spectra. These results show that not only the modulus of ε3 3 but also its phase is an important quantity when comparing experimental results with theoretical predictions.