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The triplet state processes in thioindigo have been investigated with respect to the mechanism of the trans → cis photoisomerization by laser flash photolysis. According to our experimental results the intersystem crossing takes place from the trans singlet to the trans triplet. This is also the only transient, which is detected spectroscopically. An activation energy barrier between the trans‐ and a twisted triplet determines the internal rotation. The twisted triplet decays nonradiatively to the cis and trans groundstates. A model for the mechanism of the trans → cis photoisomerization is proposed. — Preliminary results for the cis → trans photoisomerization indicate an internal rotation in the excited singlet state to a transoid configuration followed by intersystem crossing to the trans triplet from which the molecule decays into the ground state.