The IR absorption spectrum of the jet-cooled benzoic acid monomer and dimer have been recorded throughout the 500-1900 cm(-1) range via ion dip spectroscopy. Both spectra show a wealth of vibrational modes and the monomer spectrum is remarkably different from that of the dimer. Density functional theory calculations show quantitative agreement with the experimental data. The C-O-H out-of-plane bending vibration in the dimer is poorly reproduced in the theoretical calculations and a more accurate description of the doubly hydrogen bonded structure is therefore still needed. (C) 2003 American Institute of Physics.