The present invention relates to a time temperature indicator comprising at least one indicator compound in a first isomeric form, which is converted into a second isomeric form of said indicator compound in a valence isomerization reaction without migration of an atom or chemical group attached to said indicator compound in a time and temperature dependent manner, wherein the formation of the second isomeric form is detectable by monitoring a physical characteristic of the indicator. The present invention also relates to a method of manufacturing such a time-temperature indicator comprising the steps of (a) embedding in or atop a matrix said indicator compound; and (b) inducing the formation of a metastable state of said embedded indicator compound.