The paper presents the results of the investigation of the thermal stability of compounds being the components of liquid crystal mixtures. For the evaluation of the thermal stability of liquid crystals a modified method was used which consists in exposing the sample to the elevated temperature (150 degrees C) and measuring its resistivity at the same time. It was stated that the thermal stability of a compound is mainly affected by the character of bonds in the molecule and especially by the type of an end group. The most sensitive to the temperature are compounds containing the -NCS group but this cannot be an unconditional proof for the compounds to be chemically unstable at room temperature.