Surface tension of water/nitrogen and water-phenol/nitrogen systems was successfully measured by the hanging drop method in a wide domain of temperature (from 100 to 300°C) and pressure (from 4 to 30MPa), conditions little explored literature. Results show that surface tension of water-phenol mixtures decreases as phenol mass fraction increases. This decrease is observed under saturated and unsaturated conditions and is more pronounced at low temperatures and does not seem to depend on pressure. The effect of saturation on surface tension in the water/nitrogen system has been correlated with water vapor pressure by using experimental points with a great accuracy. For the water-phenol/nitrogen system, experimental data obtained with different mass fraction of phenol were correlated using Macleod-Sugden equation for mixtures. Topical heading Reaction Engineering, Kinetics and Catalysis Transport Phenomena and Fluid Mechanics