Phase transitions in thin (1.5–3 μm) planar cells of a ferroelectric liquid crystal (FLC) mixture are studied using the optical second harmonic generation (SHG) technique. This method is well known because of its high sensitivity to the structural and electronic properties of surfaces, interfaces and thin films, as well as to the appearance of the ferroelectric state of a material. It is shown that the studied FLC mixture retains a highly regular structure up to about 80 °C, when a sharp decrease of SHG intensity, associated with the transition of the LC to the nonpolar (N*) phase, is observed. SHG intensity measurements show that the transition from chiral smectic C (SmC*) to chiral smectic A (SmA*) phase which takes place at about 45 °C for the bulk FLC mixture does not influence the ferroelectric state of subsurface FLC layers. At the same time, SHG interferometry measurements show that definite changes in the FLC structure happen above the bulk transition temperature from SmC* to SmA* phase. A DC electric field induced switching of the FLC cells is also observed.