The use of sodium sulphide as an ingredient in the depilatory paint employed in the fellmongering of sheepskin is now almost universal. The sodium sulphide is in aqueous solution and is customarily thickened by the suspension of slaked lime in the solution, sufficient lime being added to make a paint which will adhere easily to the skin. This paint is applied to the inner side of the skin and its depilatory action is such as to allow the easy removal of the wool from the pelt within 24-36 hours. It is recognised that care is necessary in the making up of this paint. If the sodium sulphide solution is too strong the pelt will be damaged; in extreme cases holes will appear, leading to a burnt skin, and such cases are most common in warm weather. Although the active agent is sodium sulphide it is probable that the actual damage of a skin is due to free alkali, whether present as an impurity in the commercial sulphide or formed by the hydrolysis of the sulphide. Wilson (The Chemistry of Leather Manufacture. p. 284) to explain the mechanism of the sulphide depilation – states (1) there is a reaction between the protein keratin and the SH ion and (2) this reaction so alters the structure of the protein that the residues are more readily attacked by the OH ion. It was decided that an investigation of the alkalinity, hydrolysis, etc. of sodium sulphide would have some value. The writer undertook to investigate the use of the hydrogen electrodes in solutions of sodium sulphide in water, and in solutions of sodium sulphide to which has been added various other chemicals used in the manufacture of the depilatory paint.