SEVERAL methods for the preparation of the carbides of iron have been reported in the literature. In order to obtain a satisfactory product it may be necessary to use a relatively high temperature1, or long periods of reaction during which reaction conditions must be controlled within narrow limits for several weeks2,3. Using these methods we found considerable difficulty in preparing a sample of iron carbide which was sufficiently pure to be used in a projected study. During preliminary investigations of reactions of transition metal salts of organic acids, however, it was noticed that the solid residue from the thermal decomposition in vacuum, at about 500° C, of a ferric salt of mellitic acid gave an X-ray diffraction pattern characteristic of cementite. This communication is concerned with the use of this compound for the preparation of cementite. A detailed report of the kinetics of the thermal decomposition reaction will be published elsewhere4. Studies of the thermal decomposition of ferric salts of benzoic and of phthalic acids showed that no detectable cementite was present in the non-volatile crystalline products5.