Acid adaptation of Salmonella typhimurium at a pH of 5.0 to 5.8 for one to two cell doublings resulted in marked sensitization of the pathogen to halogen-based sanitizers including chlorine (hypochlorous acid) and iodine. Acid-adapted S. typhimurium was more resistant to an anionic acid sanitizer than was its nonadapted counterpart. A nonselective plating medium of tryptose phosphate agar plus 1% pyruvate was used throughout the study to help recover chemically stressed cells. Mechanisms of HOCl-mediated inactivation of acid-adapted and nonadapted salmonellae were investigated. Hypochlorous acid oxidized a higher percentage of cell surface sulfhydryl groups in acid-adapted cells than in nonadapted cells, and sulfhydryl oxidation was correlated with cell inactivation. HOCl caused severe metabolic disruptions in acid-adapted and nonadapted S. typhimurium, such as respiratory loss and inability to restore the adenylate energy charge from a nutrient-starved state. Sensitization of S. typhimurium to hypochlorous acid by acid adaptation also involved increased permeability of the cell surface because nonadapted cells treated with EDTA became sensitized. The results of this study establish that acid-adapted S. typhimurium cells are highly sensitized to HOCl oxidation and that inactivation by HOCl involves changes in membrane permeability, inability to maintain or restore energy charge, and probably oxidation of essential cellular components. This study provides a basis for improved practical technologies to inactivate Salmonella and implies that acid pretreatment of food plant environments may increase the efficacy of halogen sanitizers.