Spirotetrahydroindolizines (THIs) 1a-g and Spirotetrahydroazafluorenes (THF) 2a-g were previously synthesized. The high reaction rate of the thermal 1,5-electrocyclization results in the half-lives of the colored betaines 2'a-g range from 3.9 to 5.4 ns, thus showing the existence of the shortest lived colored form observed so far for pyrazoline based photochromes. However, laser flash spectroscopy was successfully used for the determination of both the half-life and absorption maxima of the colored betaines 2'a-g. Time resolved transient spectroscopy shows the geometry of the betaines 2'a-g to be related to one of the two detectable intermediates 1'a-g or 1"a-g.