The spectral induced polarization signature of porous media contaminated with non-aqueous phase liquid (NAPL) was studied. Using an accurate measurement system, the complex electrical conductivity of unsaturated porous media contaminated with either diesel fuel or motor oil at a constant water saturation was determined. Counter intuitively, the results show that replacing air with NAPL increases the real part of the complex conductivity of unsaturated porous media. We interpret the results in terms of electrochemical polarization, and suggest that polar compounds contained in the NAPL adsorb to the mineral surface leading to release of inorganic ions to the pore water, which affects both the fluid and surface conductivity. In addition, we observed a decrease in the polarization followed by further addition of NAPL, which we relate to a lower mobility of the adsorbed polar compound. This study allows a better understanding of the electrical signature of NAPL contaminated porous media, especially in variably saturated conditions.