Hemithioindigo-molecules show a photochromic behavior due to a photo-induced Z/E-isomerization on a picosecond timescale. Changes in the reaction kinetics, caused by polar substituents attached to the thioindigo-moiety, are studied using time-resolved absorption spectroscopy. The experiments reveal that substituents in the thioindigo-part influence the reaction dynamics in a different manner as previously found for stilbene-substitution. The results are discussed in the context of inductive- and resonance-effects and lead to an improved understanding of the molecular mechanisms of the Hemithioindigo-photoreaction. (C) 2008 Published by Elsevier B.V.