The presently available experimental results on the measurement of the surface tension of liquid crystals have been shortly reviewed. They are inconsistent either in view of the temperature-dependence behavior of the surface tension near the phase transition temperatures or with respect to the absolute values of the surface tension. As one of our papers to report the results obtained in our laboratory on the investigation of the interfacial phenomena of liquid crystalline substances, we reported in this first paper the remarkable time-dependence of the surface tension of a freshly formed surface, observed on several liquid crystalline substances, both of low molecular and polymeric, both in the mesophase and in the isotropic phase. After discussed several possible processes, which may lead to a time-dependence of the surface tension of a freshly formed liquid surface, we suppose that the gas sorption process taking place at the liquid surfaces may be mainly responsible for this unusual time-behavior. It has been Further suggested in the paper that such a remarkable time-dependence may occur on a liquid surface where the molecules are relatively highly ordered or have some particular structures and such ordering or structures will be influenced sensitively by the presence of certain gas molecules, perhaps either as a result of the interaction between the gas molecules and the molecules at the liquid surface or as a consequence of the gas-induced re-organization of the molecules in the liquid surface region. It is inferred that such time-dependence phenomena may have confused the measurements of the surface tension of liquid crystalline substances performed by the early workers and may have contributed to some of the inconsistencies in the obtained results so far.