The regulation of intracellular pH (pHi) in bacterial cells is achieved through control over cation (and anion) permeability. In addition to the active components of homeostasis there are contributions from essentially passive elements, such as the lipid composition of the membrane and the buffering capacity of the cytoplasm. Active homeostasis involves control over the movement of K+, Na+ and H+. Alterations in the membrane permeability for any of these cations may cause perturbation of homeostasis. In Escherichia coli this is exemplified by the controlled activation of K+ efflux systems by glutathione adducts leading to temporary acidification of the cytoplasm. This is achieved by sophisticated control over the KefB and KefC systems, and is tightly integrated with glutathione-dependent detoxification mechanisms. Such control over pHi facilitates survival of the cell following exposure to toxic electrophiles. The components of pH homeostasis will be reviewed and the molecular mechanisms, and role of, the KefB and KefC systems will be discussed.