Simultaneous laser absorption spectroscopy and laser induced fluorescence under steady-state conditions, together with analysis of fluorescence spectra at various excitation conditions, have been carried out to elucidate the photoisomerization processes of 3,3’-diethylthiadicarbocyanine iodide (DTDCI) in methanol solution. The data were obtained by varying the laser fluence at several wavelengths, and fits were performed considering two kinetic models, involving two or three isomers. The results point to the formation of a second photoisomer under high excitation fluences. Its absorption spectrum is shifted toward longer wavelengths with respect to the spectra corresponding to the normal species and the first photoisomer, which are strongly overlapped. Changes in the fluorescence spectra with the excitation fluence, appearing in the DTDCI lasing region, are attributed to the second photoisomer emission. Values are obtained for the photoisomerization and back-photoisomerization quantum yields, as well as for the thermal ground-state back-isomerization yield and ground state absorption cross sections at several wavelengths for both photoisomers.