A jet-cooled infrared spectrum of the formic acid dimer (HCOOH)(2) (FAD) has been measured by cavity ring-down spectroscopy in the C-O stretching region at 8 mu m. The low rotational temperature in the jet enabled us to observe three vibrational bands which are not resolved at room temperature. The three bands have been interpreted as the Fermi-triad system which consists of the nu(22) fundamental band and two combination bands. An analysis of the observed rotational contours led us to the first experimental determination of anharmonic force constants for FAD. (C) 2007 Elsevier B.V. All rights reserved.