IR spectra of substituted benzene compounds were measured in supercritical CO2 with an apparatus capable of in situ measurement. An experimental procedure was developed which provided reliable data and allowed quantitative analyses. The molar absorption coefficient of substituent deformation stretch and C-C ring stretch were found to vary with the system density but their density dependence was different. A method for determining the local excess density of benzene derivatives in supercritical CO2 is proposed. The expression obtained for the local excess density can be represented by a single function in terms of bulk density and exhibits a maximum just below the critical density.