The solubility data for water-ethanol-petrol-eucalyptus oil blends studied over the temperature range of − 30 to 40 °C are presented. The addition of eucalyptus oil as a cosolvent resulted in a significant depression of the cloud point temperature. Individual components of the eucalyptus oil were examined for their effect on the system, and results showed that most had little effect. Piperitone was the most effective, but 1,8-cineole was the most promising practical material. The ternary phase equilibrium data for the system water-petrol-ethanol were determined over the whole composition range at 0 and 25 °C; the addition of about 5% 1,8-cineole to petrol increased the single-phase area by more than 11%. The tie-line data were also obtained. These results suggest that eucalyptus oils, particularly those with high 1,8-cineole content, are potential cosolvents for hydrated ethanol fuel blends.