An impurity-free biologically active mixture of 2-methyl-4-isothiazolin-3-one and 5-chloro-2-methyl-4-isothiazolin-3-one, which is useful as a biocide in various products, can be prepared by reacting N-methyl-3-mercaptopropionamide or N,N′-dimethyl-3,3′-dithiodipropionamide or a mixture thereof dissolved in a first organic solvent with sulfuryl chloride dissolved in a second organic solvent, which is different from the first organic solvent, at a temperature ranging from 5 to 20° C. and, optionally, centrifuging the resulting product mixture to obtain the desired mixture substantially free of 4,5-dichloro-2-methyl-4-isothiazolin-3-one impurity.