Abstract: An apparatus and a method for monitoring the cleaning of the apparatus for at least one process container and/or at least one process line, and where the process container and/or process line can be cleaned by means of introducing a detergent solution, and with a measuring device for determining a first physical characteristic of the detergent solution, a measuring device for determining a second physical characteristic of the detergent solution, a measuring device for determining the temperature of the detergent solution, an evaluation unit which determines the detergent concentration in the detergent solution via the first and second physical characteristics and via the temperature of the detergent solution, and determines the continuance or termination of the cleaning and/or the re-concentration and/or replacement of the detergent as a function of the detergent concentration that is determined.