The photophysics of 3,3'-diethylthiacarbocyanine iodide (DTCI) in liquid mixtures of dimethyl sulfoxide (DMSO) with toluene was studied by means of steady-state and time-resolved optical spectroscopy. By variation of the diminution of the volume fraction of DMSO, the solvation shell around a DTCI species significantly decreases the quantum efficiency of its photoisomerization. The character of the fluorescence decay, observed upon excitation at 570 nm, also changes. For DMSO volume fractions larger than 14 vol %, a single-exponential function with a lifetime of about 300 ps is found; in the range from 14-8 vol % of DMSO, biexponential functions with time components of 200 and 500 ps are observed, and when the DMSO volume fraction decreases from 8 to 1.6 vol %, the fluorescence decay is again single exponential (lifetimes 500-700 ps). These findings will be explained in terms of the Rulliere model by assuming a fast equilibrium between electronically excited trans and cis isomers of DTCI.