Abstract: An apparatus for irreversibly indicating that the permissible temperature of stored products has been temporarily exceeded is described, in which a container has a heat-conducting connection to the product to be monitored and contains, in its interior, a medium having a melting point which is just above the permissible storage temperature, magnetizable particles being suspended in the medium. Moreover, solid magnetizable bodies are uniformly distributed in the container. After the deep-freezing process is complete, the magnetizable particles and the solid bodies are magnetized. If the permissible storage temperature is exceeded, the suspension becomes liquid and the pigments accumulate on the solid bodies, an optical indicator present on a wall of the container and visible from the outside detecting that the permissible temperature has been exceeded.