Tetramethyl lead (TML) has recently been adopted as an antiknock substance in gasoline. In some industries, tetramethyl lead has been substituted for tetraethyl lead (TEL) mixture or "ethylfluid". This recent development is the result of technical research which has shown that there is a more uniform octane distribution in all light and heavy fractions of gasoline in the combustion cylinders with the tetramethyl lead mixture. The TML mixture used by us in this work was put at our disposal by a local firm and is a red liquid with a characteristic aromatic odour, insoluble in water but soluble in ethyl alcohol, ether, and chloroform. It has a specific gravity (at +20°C.) of 15 and vapour pressure (at 20°C.) 36 mm. Hg. The composition of this TML mixture found on analysis is given in Table 1.