[en] Highlights: • Benzthiazoline-2-thione (BTT) is an important biological compound. • Here keto-enol tautomerization is shown unambiguously. • First time report on UV spectra shifts using quantum chemical calculations. • The lowest energy transition is π → π^∗ transition rather than n → π^∗. Benzthiazoline-2-thione (BTT) and its derivatives are industrially and biologically important heterocyclic organic compounds. Here, we present both experimental and theoretical investigations. The IR and UV spectroscopic measurements unambiguously show the keto-enol tautomerization. Here, for the first time we report spectral changes of BTT in different solvents using quantum chemical calculations and hence reproduce both the hypsochromic and bathochromic shifts in the UV spectra which are sensitive to the nature of hydrogen bonding between solvent and BTT. Our calculations also reveal that the lowest energy transition is associated with a π → π^∗ rather than a n → π^∗ transition suggested to explain experimental results.