AbstractThe photophysical properties of 3,3′‐dialkylthiacarbocyanine iodides and chlorides were measured in various solvents. It was found that photoisomerization and fluorescence are the major contributors to the deactivation of the excited singlet state; intersystem crossing occurs with only a very low efficiency. In ethanol, a triplet yield of 0.004 and a singlet oxygen quantum yield of 0.002 were determined. The photophysical parameters of these dyes are not substantially influenced by the length of the alkyl chain or the size of the halide counterion. The substitution of an ethyl with an octadecyl‐chain only slightly hinders photoisomerization, and the replacement of the chloride with an iodide reduces only marginally the fluorescence lifetimes and fluorescence quantum yields in chloroform. A significant external heavy‐atom effect is observed using dibromoethane as a solvent: triplet and singlet oxygen yields increase7–10‐fold, and the triplet lifetime decreases from 5