The pyrolysis mechanism and thermostability of the series of saturated and unsaturated fatty acids, used as molecular of organic thin solid films, were studied using a thermogravimeter. The weight loss of saturated fatty acids was caused by evaporation of the molecules, although it was accompanied by a slight decomposition of the molecules in air flow. In cases of unsaturated fatty acids, evaporation of the molecules decreased and it decomposed with an increase in the number of double bonds. The thermostability of fatty acids, measured with the initial temperature of weight loss, was enhanced with an increase in the number of carbon atoms in the case of saturated fatty acids. On the other hand, thermostability of the unsaturated fatty acids reduced with an increase in the number of double bonds. © 1990 The Japan Society of Applied Physics.