The phase behaviour of palm olein (PO) and its derivative oils (palm oil methyl esters and medium chain triglycerides) with Imbentin coco 6.9EO, an ethoxylated C12-14 alcohol, in water has been investigated to identify compositions where microemulsions occur. The techniques used were the optical microscope phase penetration scan and small angle X-ray diffraction (SAXS). Mixed surfactant/oil samples were prepared at wt. ratios of 0.1:1, 0.25:1, 0.5:1 and 1:1 for the phase penetration scan. For SAXS analysis, the initial concentration of surfactant in water (W) was fixed at 38% (w/w), which forms a hexagonal mesophase (H1). Palm oil methyl esters (POME) and medium chain triglycerides (MCT) were added to this at 0.04:1 (or 0.05:1 for MCT), 0.1:1, 0.2:1 (or 0.25:1 for MCT), 0.5:1 and 1:1 ratios of oil to surfactant. Schematic phase diagrams were constructed to document the changes of phase structures using both bulk samples and phase penetration scans techniques. The extent of microemulsion formation (or solubilisation) decreases in the sequence POME > MCT > PO, and increases substantially with temperature, particularly for POME and MCT. All of the oils destabilize the hexagonal phase; for POME and MCT there is an increase in the surfactant cloud point temperature by ca. 10 degrees C or more, but the temperature for the onset of the lamellar (Lalpha) phase dispersion region (W + Lalpha) is hardly affected. There was a pronounced tendency for the lamellar phase formed in the presence of high oil concentrations and low water levels to have a reduced melting point. With the highest MCT levels a bicontinuous cubic phase (probably V2) is present at 25 degrees C, although this phase is not present in the binary surfactant/water system. The X-ray diffraction results show that the average area per head group (ao) at the micelle surface is decreased by the addition of the oils, consistent with the observation of a V2 phase. Possible molecular mechanisms for this observation are discussed.