In his early work on the Citric Acid Cycle, Hans Krebs made measurements of oxygen consumption by suspensions of muscle cells. He found that addition of succinate, fumarate or malate to the cell suspension stimulated oxygen consumption, and that the amount of oxygen consumed was about 10-fold more than the amount needed for the complete oxidation of the added compound. Moreover, the succinate, fumarate or malate was not consumed by the cell suspension but was still detectable at the end of the experiment. What is the correct interpretation of these observations?