The real part of the complex relative electric permittivity, εr(T, p), of liquid methylbenzene has been determined with an estimated expanded relative uncertainty of ± 0.018 % from measurements of the complex capacitance of a concentric cylinder capacitor at temperatures between (293 and 393) K and pressures below 60.6 MPa. The measurements were corrected for the isothermal compressibility of the solid that formed the capacitor. The isothermal compressibility was determined by comparison of measurements of εr(He, 303 K and 393 K, p) with ab initio results from quantum mechanics. The εr(T, p) values obtained for methylbenzene were combined with the amount-of-substance density, ρ, which was obtained from the equation of state reported by Lemmon and Span (J. Chem. Eng. Data2006, 51, 785-850), to calculate the molar polarizability, P, that was fit with an expanded (k = 2) relative uncertainty of ± 0.03 % by three parameters. The εr(T, p) reported by Kandil et al. (J. Chem. Eng. Data2008, 53, 1056-1065) from measurements with a radio frequency re-entrant cavity, a technique with entirely different sources of systematic error, were converted to P·ρ and they differed from 1·10 -3 at T = 290 K to -2.5·10-3 at T = 406 K (with âŸ̈P·ρâŸ© ≈ 0.31 giving a relative difference of between 0.3 % and -0.8 %, respectively) at temperatures that overlap ours. At T = 298 K the measurements reported by Mospik (J. Chem. Phys.1969, 50, 2559-2569) at a pressure between (0.1 and 20) MPa when converted to the product P·ρ are within -0.8·10-3 (a relative difference of about -0.26 %) of our smoothing equation while at T = 273 K, about 20 K below our lowest temperature, the differences for P·ρ are -0.4·10-3 (a relative difference of about -0.13 %). © 2013 American Chemical Society.