A thermophysical study by differential scanning calorimetry (DSC) for the 2- and 3-isomers of thiophenecarboxylic acid, in the temperature intervals between T = 268 K and their respective melting temperatures has been carried out, and enhancements in the heat capacity curves were observed. The supramolecular structure of both compounds was analysed by X-ray crystallography and IR spectroscopy to determine which structural modifications take place in the temperature range at which the heat absorption occurs. Changes in bands assigned mainly to delta(O-H) + nu(C-O), delta(C-H) + nu(C-S) and out of plane C-H vibrations point to structural modifications taking place in the temperature range at which the heat absorption occurs. All those groups are involved in the weak C-H... O or C-H... S contacts that form the crystal three-dimensional network and so it can be concluded that changes in these H-bond strength, are responsible for the small heat absorptions founded in the heat capacity curves. (C) 2003 Elsevier Science B.V. All rights reserved.