Study of the effects of cinnamic, propionic, benzoic and sorbic acids on the growth and intracellular pH of E. coli suggests that the potency of weak acids as food preservatives is related to their capacity to reduce specifically the intracellular pH. Although both the undissociated and dissociated forms of the acid cause the intracellular pH to fall, growth inhibition may be due predominantly to the undissociated acid.