This work reports on the experimentally measured densities, viscosities, speeds of sound, surface tensions, and flash points, and calculated bulk moduli of mixtures of 2,2,4,6,6-pentamethylheptane (iso-dodecane) with iso-butylbenzene, sec-butylbenzene, tert-butylbenzene, or 1,3-diethylbenzene. As the mole fraction of the aromatic component increased, the viscosities decreased and the densities, speeds of sound, bulk moduli, surface tensions, and flash points increased, except for mixtures with tert-butylbenzene, where the flash point did not vary because both components had the same flash point. The excess molar Gibbs energies of activation for viscous flow were slightly negative (but not statistically different from zero), and the viscosity deviations were negative at 293.15 K. At higher temperatures, the viscosity deviations were closer to zero. Excess molar volumes V-m(E) for these nonpolar mixtures were positive for iso-butylbenzene, sec-butylbenzene, and tert-butylbenzene and negative for 1,3-diethylbenzene, suggesting that the more bulky branched substituents on the aromatic packed less well with the iso-dodecane than did the disubstituted linear substituents. The values of V-m(E) did not vary over the temperature range measured. For all mixtures, excess speeds of sound were positive and excess isentropic compressibilities were negative at 298.15 K. Knowing the impact of various isomers on the physical properties of mixtures can enable researchers to select the best compound for use in their design of surrogate fuel mixtures.