Vapor and sublimation pressures of three n-alkanes C-20, C-24, and C-28 have been determined using a modified gas saturation method. The obtained pressure values range from 10(-5) Pa to 5.5 Pa. From the temperature dependence of the vapor pressures, the molar enthalpies of vaporization and sublimation at the mean temperature of the experimental range were derived from the Clausius-Clapeyron equation. From these results, the standard enthalpies of vaporization and sublimation at T = 298.15 K were calculated.