Laser Hash photolysis, laser induced optoacoustic spectroscopy (LIOAS), and fluorescence data were used for the study of the photophysical deactivation processes of excited 3,3 diethylthiadicarbocyanine iodide (DTDCI). Direct excitation of this molecule in air saturated methanol solutions at room temperature results in the formation of only one transient species, a photoisomer, the spectrum of which was derived for the 600-680 nm spectral range. The energy content difference between the ground state of the stable form and that of the photoisomer was evaluated, $latex \Delta E = 0.15 \pm 0.15 \enspace eV$, as well as limit values for the photoisomer fluorescence quantum efficiency $latex(\Phi _{f} ^{P}) < 0.2$ and for the quantum yield for the back photoisomerization from the first excited singlet state of the photoisomer to the ground state of the stable form $latex(\Phi _{PN}\leq 0.14)$.