Supercritical CO2 has many unusual physicochemical properties in or near the critical region, which is mostly due to the local density enhancements and specific molecular interactions. Compared to pure CO2, the study on CO2 mixed with co-solvent is attractive in wide applications but it is hard to quantitatively describe the local density change and intermolecular clusters in CO2 mixtures. The fluorescence response of a sensitive pyrene probe, empirical Py scale, was determined in CO2 mixed with pentane at 323.15K in different phase regions. A variable called the order parameter was introduced to account for possible correlations in the first derivative of Py scale and fluid pressure (dPy/(d(PIP0 - 1))). This model, avoiding the hard choice of the reference line in CO2 mixtures, was effective in calculating the local density change and it showed the specific molecular interaction in the critical region. The behaviors of the local density change and the isothermal compressibility of supercritical CO2 mixtures correlated well with microscopic and macroscopic observations. (C) 2010 Elsevier B.V. All rights reserved.