The formulation of macro- and microemulsions through the Hydrophilic Lipophilic Deviation (HLD) concept based on phase behaviour observations, has mostly been applied to mineral oils. The present research reports a similar approach using triglyceride-based oils (seven vegetable oils and one pure triglyceride) and ethoxylated (nonylphenol and isotridecanol) surfactant systems. Formulation scans are carried out by varying the surfactant average degree of ethoxylation, eg the ethylene oxide number (EON), at 50 °C, 55 °C and 60 °C. Because these systems do not exhibit a Winsor III phase behaviour, the so-called optimum formulation was detected for EON0.5, ie the EON value at which the surfactant partitions equally into the aqueous and the oil phases, which was found to coincide with the minimum stability of the emulsified systems. Experimental EON0.5 values ranged from 3.5 to 5.5 as temperature, oil nature and surfactant were altered. © 2003 Society of Chemical Industry