Photophysical properties of 1-(2'-Hydroxy-5'-Chlorophenyl)-3-thio-5-oxo-1H-imidazo-[3,4-b] isoindol (HCHT) have been investigated thoroughly. Steady state and time resolved absorption and emission measurements in combination with theoretical calculation reveal that the synthesized molecule HCHT shows excited state thiolactim-thiolactum tautomerism through intramolecular hydrogen bonding between donor sulphur atom (-S-H) and acceptor nitrogen (N) atom. This is by far the first example where a sulfur atom is present as donor in an excited state proton transfer (ESIPT) process. Choice of sulfur atom in HCHT decelerates the irreversible ESIPT process compared to its oxygen atom donor analogue as evidence from excited state lifetime experiments.