AbstractEquilibrium constants (K) and thermodynamic parameters for the charge‐transfer complexes between diiodine and Δ3‐4‐S‐methyl‐5,5‐dimethylimidazolidine‐2‐thione (1), 2‐S‐methyl‐5,5‐dimethylimidazolidine‐4‐thione (2), and Δ1‐Δ3‐2S‐methyl‐4‐S‐methyl‐5,5‐dimethylimidazolidine (3) have been determined in CH2Cl2by UV‐Visible spectroscopy. Depending on the solvent polarity, compound2exists as a Δ1or Δ2tautomer; both the tautomers are present in CH2Cl2. Compounds1and2act as good donors (K = 11,000 ± 800 and 640 ± 21 dm3mol−1, 25°C, respectively) owing to the presence of a thioamidic group in the molecule, whereas in compound3, the N‐coordination causes a strong reduction of the formation constant (K = 14.6 ± 0.8 dm3mol−1, 25°C). The high difference between the stability constants of1and2has been ascribed to an intramolecular hydrogen bonding between NH and the bonded iodine, which can be possible only for1. Additional spectroscopic data (FT‐Raman, NMR) enables a deeper unde