The diffusion coefficient of methyl orange in dense carbon dioxide (CO2) has been measured from (87.6 to 226) bar and from (20 to 40) degreesC. A hydrodynamic model and the hard sphere diffusivity model have been fit to the measured data. The effect of the micelle-forming, nonionic, nonfluorous surfactant Dehypon Ls-54 was also investigated. Effective diffusivity of methyl orange in the CO2 + Ls-54 + H2O system was measured at 35 degreesC and 220 bar at Ls-54 concentrations up to 0.165 M. A simple volumetric-based model based on the binary diffusivity data fits the effective diffusivity well, given realistic property estimates.