New experimental results are presented for the solubility of oxygen in the ionic liquid 1-N-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) for temperatures from (293.2 to 373.1) K and pressures up to 9.1 MPa. The ionic liquid [bmim][PF6] is a poor solvent for oxygen; for example, at 9 MPa only about 0.16 mol of the gas are dissolved in I kg of the ionic liquid. Nearly no influence of temperature on the solubility of oxygen in [bmim][PF6] was observed. The (extended) Henry's law is applied to correlate the solubility pressures. Furthermore, prediction results from Gibbs ensemble Monte Carlo simulations (at constant pressure and temperature, i.e., NpT-GEMC) are reported. The intermolecular forces are approximated by an effective pair potential for oxygen (the two-center Lennard-Jones model proposed by Coon et al.) and by a quantum-chemistry based pair potential for [bmim][PF6] (the UA1 model by Shah et al.). The interactions between unlike groups are calculated by applying common mixing rules (without any adjustable binary parameters). (c) 2005 Published by Elsevier Ltd.