The importance of diacetyl from the standpoint of the desirable aroma of butter has been shown by various investigators. van Niel, Kluyver and Derx (6) found from 0.0002 to 0.0004 percent diacetyl in fine butter, and when these concentrations of diacetyl were added to butter neutral in odor an unmistakable aroma appeared. The important conclusion of these investigators was that diacetyl is either responsible for the aroma of butter or is the principal component of the aroma material. Schmalfuss and Barthmeyer (4) found diacetyl to be an aroma constituent of various materials, including butter. The diacetyl was considered to come from acetylmethylcarbinol. Margarine to which diacetyl had been added took on the aroma of butter. Four samples of butter, representing different conditions of feeding the producing animals, yielded from 0.0001 to 0.0006 grams of nickel salt (equivalent to diacetyl) per kilogram, and the quantity of diacetyl in the butter appeared to be correlated with the intensity of the aroma.