A full-history time-temperature indicator system which is capable of exhibiting a time-temperature dependent and visually detectable chemical reaction, useful for monitoring the time and temperature exposure of food products. The system comprises an immobilized reactant, such as Fe.sup.3+, and a mobile reactant, such as Fe(CN).sub.6.sup.4-, initially contained in separate compartments, and separated by a sealing. The system is activated by removing a sealing between the compartments whereby the mobile reactant in a time-temperature dependent manner is brought into contact with the immobilized reactant resulting in a visually detectable reaction signal.