We have investigated the effect of ice substrate on the temperature dependent structural changes of formic acid. Infrared spectra of HCOOH@KBr, HCOOH@H2Oamorphous and HCOOH@H2Ocrystalline were measured in the temperature range of 18-180 K (X@Y represents X deposited on Y). At 18 K formic acid deposited over ice exists principally as its dimer. Upon increase of temperature, the dimer gets converted to the polymeric crystalline form. In the case of amorphous ice surface, this conversion starts at a lower temperature of 37 K. For KBr and crystalline ice surfaces, this occurs at 98 and 83 K, respectively. The dangling -OH found on amorphous ice surfaces disappears completely upon formic acid deposition. The greater mobility of formic acid molecules on amorphous ice aids in the early onset of its crystallization. (C) 2004 Elsevier B.V. All rights reserved.