A new method to measure the spontaneous polarization density PS of ferroelectric liquid crystals as a function of the temperature is described. It is based on the spectral decomposition of the polarization and measurement of the different temporal harmonics using a lock-in amplifier. It is particularly adapted to measure small values of PS in regions where different nonlinear process compete. This condition is fulfilled at temperatures around the SmC*–SmA transition temperature in liquid crystals. The method has a quantitative procedure to evaluate the saturation state of the sample’s polarization at a given applied electric field. It overcomes the difficulty of the subjective evaluation of the hysteresis loop present in the usual methods. The accuracy in the measurement of PS of the ZLI-3654 ferroelectric liquid crystal in the region of about 0.5 nC/cm2 is approximately one order of magnitude larger than that obtained with conventional hysteresis loop methods. © 2000 American Institute of Physics.