The effect of temperature on microemulsion formation was studied in C12H25O(CH2CH2O)4H/alkanes/water systems. A three-phase region composed of water, alkane, and microemulsion phases appeared with rise in temperature. The temperature, at which a middle phase microemulsion forms, rose as the alkane carbon number was increased. The diameter of the dispersoid particles in the middle phase microemulsion changed as the temperature was raised. At the temperature which gave minimum diameter of the droplets, moisture content registered 50% and the density intermediate between those of water and oil. It was found that the diameter of the dispersoid particles in the middle phase microemulsion is minimized at the temperature at which hydrophilic-lipophilic properties of the system reach perfect eqilibrium.