THE concept of intracellular pH depends on the assumption that the cell is either not divided into compartments, or that the volumes of the compartments are large enough to contain a statistically meaningful number of hydrogen ions, the “intracellular pH” in this case being a mean of the pH values in the various compartments. While this may be valid for cells, the concept cannot necessarily be extended to very small intracellular compartments. This can be shown in the case of a mitochondrion of say 0.5µ diameter and 5µ length. The volume of this compartment would be approximately 10−12 ml. At pH 7.0 the hydrogen ion concentration is nearly 10−7 g ions/l., and thus the mitochondrion volume would contain 10−7 × 10−3 × 10−12 g ions. If the Avogadro number is 6 × 1023, then the mitochondrion will contain that is, thirty ions.