BACKGROUND Skin exposure to biocides containing high concentrations of methylchloroisothiazolinone (MCI) and methylisothiazolinone (MI) may cause both severe chemical burns and induce sensitization. OBJECTIVE This study was performed to investigate whether sodium bisulfite, as a model substance, could be used for treatment of skin accidentally exposed to high concentrations of MCI/MI to prevent chemical burns and elicitation of allergic contact dermatitis. METHODS The reaction between Kathon WT and sodium bisulfite was studied with chemical methods. MCI/MI-hypersensitive patients and controls were patch tested with serial dilutions of Kathon CG and with reaction mixtures being about 30 seconds, 30 minutes, and 48 hours old. RESULTS The concentrations of MCI/MI rapidly decreased after addition of sodium bisulfite. The degradation products did not elicit any irritant test reactions neither in test patients nor in controls. Five out of seven Kathon CG-hypersensitive patients reacted to a reaction mixture about 30 seconds old and containing MCI/MI at 14 ppm, whereas four and two individuals tested positively to the reaction mixtures being 30 minutes and 48 hours old, respectively. The controls tested negatively to all reaction mixtures. All tested persons reacted negatively to sodium bisulfite. CONCLUSIONS The deactivation of MCI/MI with sodium bisulfite occurred instantly and it prevented corrosive skin reactions and the elicitation of allergic contact dermatitis was decreased substantially.