Fourier transform infrared (FTIR) spectra and hydrogen bonding of acetic acid in CO2 (1) + n-pentane (2) mixture were studied at 308.2 K up to 11 MPa. The experiments were conducted in the mixed supercritical (SC) and subcritical fluids near critical region and far from the critical region. The results demonstrated that the frequency of C=O stretching vibration of the monomer and dimer, the molar absorptivities of monomer and dimer, and the monomer-dimer equilibrium were very sensitive to pressure in the subcritical fluids and supercritical fluids in the critical region, especially as the pressure approached the phase boundary, while the effect of pressure on these properties outside the critical region was very limited. (C) 2003 Elsevier B.V. All rights reserved.