infrared spectra of jet-cooled 5-methoxysalicylic acid in the 3-mum region were measured for both the electronic ground (S-0) and first excited (S-1) states. Effects of the methoxy-substitution were investigated in relation to the excited-state intramolecular proton transfer of salicylic acid. Though the electronic absorption-emission spectra exhibit the mirror-image relation, which has been regarded as evidence for the suppression of excited-state intramolecular proton transfer, the infrared spectra showed an extremely large elongation of the phenolic O-H bond upon photoexcitation. The results represent that the significant enhancement of the intramolecular hydrogen bond does occur in S-1 of 5-methoxysalicylic. (C) 2003 Elsevier B.V. All rights reserved.