Phase transition pressures of the supercritical carbon dioxide (scCO(2)) + polyethylene-2,6,8-trimethyl-4-nonyl ether (TMN) systems were measured, using a synthetic method, at temperatures from 313 to 343 K. Three polydisperse TMN samples with different average molecular weights, due to the varying numbers of ethylene oxide groups in the molecules, were used in the experiment: TMN-10, TMN-6 and TMN-3. The molecular weight distribution of each TMN was determined using a mass spectrometer with a time of flight detector. The (M) over bar (n), (M) over bar (w) and (M) over bar (z) values [g/mol] were 718.8,746.9 and 775.7 for TMN-10, 624.2,646.8 and 668.1 for TMN-6, and 397.5, 409.4 and 421.2 for TMN-3, respectively. The phase transition pressures increased with increasing temperature for all systems, and also increased as the average molecular weight increased at a constant temperature and TMN weight fraction. A new prediction model based on the Sanchez-Lacombe equation of state was developed to predict the phase boundaries by introducing the group contribution method. The group parameters, which are the segment interaction parameter, the number of segments per TMN molecule, and the Volume occupied by one segment, were determined by correlating the experimental phase transition pressures for the scCO(2) + TMN-10 and the scCO(2) + TMN-6 systems. Although some deviations were observed between the correlated and experimental results of the scCO(2) + TMN-10 system due to the inaccuracy of the correlation of the temperature dependency of the phase transition pressures, the correlated results for both systems reproduced the experimental results moderately well. The predicted results were also in good agreement with the experimental results for the phase transition pressures of the scCO(2) + TMN-3 system. (C) 2009 Elsevier B.V. All rights reserved.