Abstract: A telltale device for perishable products including a layer of temperature sensitive material having a clearly defined melting point and an indicator layer separated by a diffusion barrier formed of natural and synthetic gels which permits diffusion of the temperature sensitive material through the barrier at a controlled rate for contact with the indicator after a prescribed period of time at or above the melting point of the material. The temperature sensitive material and diffusion rate of the barrier are selected so that the device, when affixed to a product, visually indicates if the product has been held at or above its deterioration temperature for sufficient period to affect the quality of the product.