Effects of dissolved carbon dioxide (CO2) on the solvation dynamics of coumarin 153 were studied in 1-butyl-3-methylimidazolium hexafluorophosphate, and 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl) amide by the time resolved fluorescence spectroscopy. The solvation dynamics showed an ultrafast response less than 1 ps and a slower response extended to a half of ns under ambient condition. With increasing the CO2 pressure, the slower component became fast, although no significant change was observed for the faster component. The CO2 effect on the average solvation time was compared with that on the translational diffusion of the solute molecule dissolved in the mixture. (C) 2011 Elsevier B. V. All rights reserved.