A new chemical sensing system using an electrical oscillator has been developed. This sensing system measures the electrical ‘non-linearity’ at the surface of an electrode immersed in a test solution: a sinusoidal voltage is applied to the electrode and the higher harmonics of the output current are obtained by Fourier transformation. This sensing system has been used to detect and quantify surfactant molecules in solutions. The relative intensity P2/P1 of the peaks of the second (P2) and first (P1) harmonics in the output current was found to be linearly correlated with the logarithms of the concentrations of cationic surfactants such as cetylpyridium bromide (CPB) and cetyl-N,N,N-trimethylammonium bromide (CTAB), but not with those of the anionic surfactant sodium dodecyl sulfate (SDS) or the neutral surfactant Triton X-100. The reproducibility of this sensing system was shown to be excellent.