Although they have been known since the early 2000s, it was not until recent years that deep eutectic solvents and eutectic mixtures began to be systematically studied as cheaper and more environmentally friendly alternative solvents. The introduction of hydrophobic, all-natural, neutral eutectic mixtures affords new applications, which need accurate measurements of thermophysical properties. In this work, the surface tension of dl-menthol:octanoic acid eutectic mixtures was studied over the whole range of composition from 298.15 to 333.15 K. The effects of composition and water on the surface tension are accessed and discussed. Densities and viscosities were also measured from 293.15 to 353.15 K at atmospheric pressure.