The unstable systems formed by phenols or amines and the free radicals which are produced as their immediate oxidation products have been characterized by a new term called the critical oxidation potential. This is the potential of some oxidizing solution (a mixture of ferri-and ferrocyanides, or of the cyanides of molybdenum or tungsten) which will cause a certain small amount (0.5%) of the phenol or amine to become oxidized in 5 min. when equivalent amounts of the sample and the oxidizing agent are employed. This value, which may be determined with reasonable accuracy, represents the potential at which the velocity of oxidation becomes vanishingly small, and hence the result is influenced by the reaction rate to an insignificant extent. This has been demonstrated experimentally, for the critical oxidation potential, E c, bears a definite relationship to the normal potential, E o. Thus in a neutral alcoholic buffer solution at 25[degree] these results were obtained (system named as reductant): p, p[image]-dihydroxy-diphenyl: E c, 0.882v.; E o, 0.954v. p, p[image]-dihydroxystilbene: E c, 0.786v.; E c, 0.854v. p -phenylene-diamine: E c, 0.710v.; E o,0.783v. p-amino-phenol: E c, 0.673v.; E o, 0.733v. Assuming the same average difference (0.068v.) to apply in other cases, normal potentials were calculated for systems which are too unstable to permit direct measurement. Thus E o for the systems (named as oxidant) are: o-ben-zoquinone, 0.810v.; amphi-naphthoquinone, 0.758v.; 3-hydroxy-l, 2-benzoquinone, 0.677v. It is assumed that with monatomic phenols or amines \E o is 0.136v. above E c. For phenol it is estimated that E o is 1.225 v. and that the free energyof oxidation to the free phenoxyl radical is 28.3 Cal. The critical oxidation potential of phenol is increased by ortho or para substitution of an unsaturated group (0.344v. for NO2), and decreased by similar substitution of a saturated group ( [long dash]0.371v. for N(CH3)2). [beta]-Naphthol and [beta]-phenanthrols do not differ appreciably from phenol; but E c for [alpha]-naphthol and [alpha]-phenanthrols is nearly 0.3v. lower than that of phenol. Primary amines are about 0.04v. higher in oxidation potential than the corresponding phenols; diphenylamine has a potential of l.OO8v. as compared with 1.135v. for aniline. Many other comparisons and theoretical deductions are presented in the original paper.