Kathon CG was determined in 156 of the most commonly used cosmetic products in Denmark. 42% of the cosmetic products were found to contain Kathon CG. Kathon CG was present in 48% of "rinse off" and 31% of "leave on" cosmetic products. The 3:1 ratio of chloromethyl isothiazolinone (CMI): methyl isothiazolinone (MI) in Kathon CG was found to be disturbed in 41% of Kathon CG preserved cosmetic products. The disturbed CMI/MI ratio was shown in 29% "rinse off" and 73% "leave on" cosmetic products. It is suggested that the chemical reaction of the ketone group of the isothizaolinones with other ingredients in cosmetic products may disturb the CMI/MI ratio of Kathon CG. The results of the study have been discussed in relation to the use of Kathon CG in cosmetic products as well as test material for allergic reaction.