Abstract: A time/temperature device for monitoring and indicating variations in the ambient temperature of a space being monitored above a predetermined temperature level is encapsulated in a plastic bag, and uses a carrier to be impregnated with a fusible diffusing medium having a sharp melting point at the predetermined temperature being monitored adapted above the melting point to change from the solid phase to substantially the molten or liquid phase and vice versa below the melting point, a dye composition is formed into a mixture in a given concentration so that it can be coated at one end of the carrier, and the concentration will be in ratio to the volume of the diffusing medium so that the dye composition has a finite and reproducible diffusion rate through the diffusing medium whenever the diffusing medium is in the liquid state and exhibits no diffusion when the diffusing medium is in the solid state, and an indicating section on the face of the carrier for the time/temperature device will provide a visible