Abstract The time constant for color formation with temperature change was studied by applying a pulse of heat to films of thermochromic salicylidene-anilines. There is a weak absorption band formed around 5400 Å, in which the time constant for color formation is about 50 msec. For the larger absorption band, around 4800 Å, the time constant was shorter than the sensitivity of our instrument, i.e. shorter than 5 msec. This indicates the mechanisms are different for the two bands. Both are probably intramolecular processes, the 4800 Å band being caused by tautomerization to a quinoid state, while the 5400 Å band is due to another species whose exact nature is not well understood.