Temperature indicator for refrigerated products operating on the principal of solidification and melting of liquid bodies at room temperature is provided with a transparent envelope containing a capsule filled with an aqueous solution which is liquid at room temperature. The capsule includes a fracture point causing it to split into two parts due to solidification of the solution, and further includes a compressed spring positioned so that when the capsule is fractured, the spring remains compressed as long as the solution is in the solid state, and visibly and irreversibly separates the two parts of the capsule if the solution turns to a liquid. The monitoring device serves to verify that the temperature is maintained either between two limits or below or above a critical temperature.