The phase behavior of a binary mixture consisting of a supercritical fluid and an imidazolium-based ionic liquid was studied experimentally. Carbon dioxide and 1-hexyl-3-methylimidazolium hexafluorophosphate were the selected supercritical fluid and ionic liquid, respectively. A synthetic method was used to measure vapor-liquid boundaries. Results are reported for carbon dioxide concentrations ranging from 9.8 to 72.7 mol% and within temperature and pressure ranges of 298.31-363.58 K and 0.64-94.60 MPa, respectively. The experimental results obtained in this work were compared with phase behavior data of the binary system of carbon dioxide together with another member of the same homologous ionic liquid family (1-ethyl-3-methylimidazolium hexafluorophosphate). The CO2 + [hmim][PF6] binary system has a two-phase region liquid-vapor extending up to very high pressures. (C) 2003 Elsevier B.V. All rights reserved.