Binary diffusion coefficients of isobutylbenzene, sec-butylbenzene, and 3-methylbutylbenzene (isopentylbenzene) in carbon dioxide were measured at supercritical conditions: temperatures of (313, 323, and 333) K and in a pressure range of (15.0 to 35.0) MPa using the Taylor-Aris technique. The dependency temperature, pressure, carbon dioxide density, and carbon dioxide viscosity was examined. The experimental results were compared with the predictions of 14 semiempirical equations derived from the Stokes-Einstein equation (six equations) and from the model of the rough-hard-sphere (eight equations). Despite of the low errors of some of them, none reproduced the trend in which the isomers diffused.