A novel keto-enol phototautomeric compound of 1-phenyl-3-methyl-4-(-furoyl)-5-pyrazolone 4-methyl thiosemicarbazone was found to undergo phototautomerization in the crystalline state, The reaction rate constant was studied based on the first-order kinetics curve. Crystal structural analysis and theoretical calculations show that the pyrazolone ring stabilizes in the keto form. The conclusion can be made that its phototautomerization in the crystalline state is associated with a photo-induced intermolecular double-proton-transfer reaction along intermolecular hydrogen bonds N-(HO)-O-... and (SH)-H-...-N leading to a colored tautomer as the compound crystallizes in a hydrogen bonded supramolecular configuration. (c) 2006 Elsevier B.V. All rights reserved.