5-Chloro-2-methyl-4-isothiazolin-3-one

5-Chloro-2-methyl-4-isothiazolin-3-one (CAS 26172-55-4), commonly abbreviated as MCI or methylchloroisothiazolinone, is a synthetic isothiazolinone biocide supplied as an aqueous liquid. It is commercially known under trade names including Kathon WT and Kathon CG 5243, and is registered under EC number 247-500-7.

MCI functions by disrupting microbial cell respiration and DNA synthesis, making it effective against bacteria, fungi, and algae at low concentrations. In water treatment and cooling systems, it controls biofilm formation. In rinse-off personal care formulations and industrial coatings, it prevents microbial spoilage during storage and in-use contamination.

MCI is supplied as a liquid concentrate, typically in combination with MIT (methylisothiazolinone), and is subject to REACH registration requirements in the EU. In cosmetics, maximum permitted concentrations are regulated under EU Cosmetics Regulation 1223/2009. Packaging is available in drums and intermediate bulk containers.