The flesh of fresh apricots when cut and exposed to the air rapidly develops a brown color due to enzymic oxidation. This phenomenon is of great importance in the freezing and freezing storage of fresh apricots and in the preparation of apricots for canning and drying. In commercial freezing the oxidizing enzymes of this fruit are controlled by treatment with dilute SO2 solution or by covering the fruit with sirup containing ascorbic acid. In canning the enzymes are destroyed by heat, and in drying they are inhibited by S02. Sumner and Somers (11), Onslow (5) and others have reviewed the early research on the oxidizing enzymes of plant tissue. Therefore, we shall cite only a few papers that bear more or less directly on fruit oxidases and refer the reader to texts such as the above for a more comprehensive review. Samisch (8, 9, and 10) who conducted research on apricot oxidase in this laboratory used a Barcroft manometric apparatus with catechol as the substrate in studying certain properties of this enzyme or enzyme system. He showed that phenolase activity was greatest in the peel and vascular bundles. He investigated the effect of pH value and various enzymes on apricot phenolase, using catechol as the substrate. Potatoes possess a very active oxidase, classified by Boswell and Whiting (1938) as a polyphenolase. Samisch (8) defines polyphenolases as ' ' oxidases which have the power to catalyze the oxidation of phenols or their derivatives by molecular oxygen.'' Peroxidases do not have this power and require the presence of H202. However, Szent-Gy?rgyi (12) suggested that peroxidases are merely attenuated polyphenolases that have lost the power of catalyzing the oxidation of phenols by atmospheric oxygen. Cruess and Fong (2) showed that the inactivation temperature of fruit peroxidase varies with the hydrogen-ion concentration of the medium; that there is an optimum H202 concentration for fruit peroxidases, and that the optimum pH value varies with the oxidase indicator used; i.e., optimum pH appears to be a function of the substrate rather than of the