We report the mid-infrared absorption spectrum of 5,6-dihydroxyindole-2-carboxylic acid (DHICA) solid samples in the wavenumber region ranging from 1000 cm(-1) to 4000 cm(-1) with resolution of 4 cm(-1). Vibrational modes for observed dominant absorption bands of DHICA have been successfully assigned with ab initio density-functional theory calculations. The experimental data also suggest the existence of intermolecular hydrogen bonds. (c) 2006 Elsevier B.V. All rights reserved.