Abstract: Background Methylisothiazolinone (MI) contact allergy is severely affecting consumers with allergic contact dermatitis, owing to its presence in cosmetics, household detergents, and water-based paints, in particular. Data on the true isothiazolinone concentrations in these products are scarce, and labelling may be incorrect. Objectives To report on the MI concentrations in such products marketed in Belgium, in order to verify the accuracy of labelling (when applicable) and compliance with EU regulations. Materials and methods Thirty cosmetics (18 leave-on and 12 rinse-off), eight detergents and four paints were analysed for MI by the use of high-performance liquid chromatography with ultraviolet detection. Results The analysed leave-on, and to a lesser extent the rinse-off, cosmetics, contained MI at concentrations far exceeding the permitted 100 ppm use concentration. Household detergents contained high concentrations of MI, and mislabelling occurred for both cosmetics and detergents. The (limited) data on paints are in line with the existing literature. Conclusion Cosmetics and detergents may facilitate contact sensitization because of a (too) high MI concentration, and mislabelling may make its avoidance extremely difficult. Safer use concentrations and correct labelling should be ensured by adequate quality control.