Ferricytochrome a (a 3+) (418,600 m[mu]) was reduced to ferrocytochrome a(a 2+) (444,605 m[mu]) by ascorbic acid or p-phenylenediamine. In the presence of O2 a2+ was oxygenated to a complex compound, a2+ -O2 (426-428,603 m[mu]). a2+ -O2 was rapidly decomposed by Na2S2O4 to a2+ and O2, and was gradually autooxidized to a3+. This autooxidn. of a2+-O2 was accelerated by ferrocytochrome c(c2+), and to a lesser extent, by ferricytochrome c(c3+), and gave rise to another type of a3+(a3+ *), as indicated by the spectrophotometric studies of the transient formation of a3+ *. A mechanism of terminal oxidn. processes, including the functional state of a, has been presented a3++ e(electron from the reductant) [forward arrow] a2+; a2+ + o2 [forward arrow] a2+ -o2; a2+ -O2[long dash](mediated by c3+)[forward arrow] a3+ * ; a3+# + e [forward arrow] a2+.