Abstract: The invention concerns a method of determining the concentration of an iodine-iodide-containing active substance in aqueous solutions intended for use in the cleaning or disinfection of containers such as bottles, drums, kegs, cases and tanks and/or pipe lines in the food-processing industry, as well as for industrial scale cleaning in continuous washing plants, the concentration of the active substances in the solution being measured by determining the iodine/iodide content of the solution. The invention provides a rapid, reliable and accurate method which is not prone to interference and can be used on a continuous basis. In addition, it makes it possible to automate the process by which the active substance is kept at the required concentration. In particular, the method proposed can be employed in an integrated cleaning-in-place system.