The rates of oxidation of N-methylacridan by 2,6-dichlorophenolindophenol have been measured between pH 2.74 and 6.91 at 20 to 22 °C in the presence of 8 to 22 per cent. ethanol. The reaction was of the second order and was found to proceed by ' two simultaneous mechanisms, both involving the conjugate acid of N-methylacridan. The oxidation of this cation by the indophenol anion proceeded at a rate which was 19 times greater than the oxidation by the uncharged indophenol molecule. It is shown that oxidation probably occurs by hydrogen atom transfer rather than electron transfer. A similar mechanism for the oxidation of dihydro-coenzyme I would account for its slow reactivity towards molecular oxygen and the biological necessity for mediating systems involving both hydrogen atoms and electrons.