A bipolar membrane (BM) is composed of one cation and one anion ion-exchange layers joined together in series. In order to obtain the AC electrical impedance of a BM, a small sinusoidal current perturbation was superimposed to the DC current, and the resulting frequency-dependent impedance spectra were recorded under different conditions of electrical polarisation and temperature for five BMs. The experimental spectra were measured in three current ranges : below the limiting current region, at the onset of the overlimiting region and in the electric field enhanced water dissociation region. This allows for a better understanding of the contributions of the salt and water ions to the measured impedance spectra. Measurements of the impedance of the forward biased membrane were also carried out. Although the experimental impedance spectra appear to be in qualitative agreement with previous theoretical models incorporating the effect of the electric field enhanced water dissociation, a quantitative analysis of the results is not still possible due to the high number of parameters involved.