The diffusion coefficients (D) of ferrocene have been determined by chronoamperometry in two ionic liquids (ILs), 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([BMIM][Tf2N]) and 1-methyl-3-octylimidazolium bis(trifluoromethanesulfonyl)amide ([OMIM][Tf2N]) at atmospheric pressure and mixed with CO2 at high pressures (p = 1.5, 3.0, and 4.5 MPa) at two isotherms (T = 298.15 and 313.15 K) using the high-pressure electrochemical cell developed in our laboratory. The results showed that D increased with increasing pressure in two ILs, of which the dependence was well represented by a quadratic form of pressure. More remarkable increase in D with pressure was observed at lower temperature in both ILs, due to the higher solubility of CO2 at a fixed pressure. This leads to the crossing points in the plots of D vs. p at two different temperatures, which correspond to similar to 3.1 MPa for [BMIM][Tf2N] and similar to 3.5 MPa for [OMIM][Tf2N]. (C) 2009 Elsevier B.V. All rights reserved.