The thermal back isomerization of 5,5',6,6'-tetrachloro-1,1',3,3'-tetraethylbenzimidacarbocyanine cis photoisomers studied by nanosecond laser flash photolysis is reported in polar solvents and at different temperatures. The data show the formation of a cis isomer after laser excitation, which disappears afterwards in the microsecond timescale with first order kinetics, forming again the trans conformer. The reaction rate constant does not depend on the solvent viscosity, but exhibits strong polarity dependence. Accordingly, the activation energy exhibits a parabolic polarity dependence, which is attributed to the charge transfer nature of the transition state. Such effect may be enhanced by stereochemical hindrance in the cis photoisomer. (C) 2002 Elsevier Science B.V. All rights reserved.