Proton transfer processes of 4-hydroxy-3-formyl benzoic acid (HFBA) have been investigated by means of absorption, emission and nanosecond transient spectroscopy in some nonpolar and polar aprotic solvents at room temperature and 77 K. The excited state intramolecular proton transfer (ESIPT) is evidenced by a large Stokes shifted emission (similar to 12 000 cm(-1)) in nonpolar solvents which is likely to originate from the enol tautomer of HFBA. On the other hand, intermolecular interaction occurs in proton accepting polar aprotic solvents both in the ground and excited state. At 77 K, HFBA shows phosphorescence in DMSO. It is proposed that occurrence of phosphorescence is due to the rupture of intramolecular bond and rotation of the formyl group. From the nanosecond measurements and quantum yield of fluorescence, we have estimated decay rates in polar solvents. (C) 2003 Elsevier Science B.V. All rights reserved.