The self-diffusion coefficients of the cation and anion species in 1-butyl-3-methylimidazolium hexafluorophosphate, [BMIM] [PF6], were independently determined over a wide temperature range by observing H-1 and F-19 nuclei with the pulsed-field gradient spin-echo NMR technique. The self-diffusion measurements were carried out for two samples of [BMM][PF6], of which one was synthesized in our laboratory and the other was purchased commercially (97%). There has been a relatively large difference observed in the self-diffusion coefficients between the two samples. It has become apparent that a small amount of impurity significantly affects the transport phenomena in the ionic liquid. On the basis of the hydrodynamic model, moreover, the self-diffusion coefficients of [BMIM][PF6] were discussed in terms of intermolecular interactions. (c) 2004 Elsevier B.V. All rights reserved.