A surfactant has been found that is effective in preventing phase separation of alcohol-petroleum fuel mixtures. It is described herein, along with the test procedures used to determine its effectiveness. These methods included exposing samples to the atmosphere until phase separation occurred; adding water to prepared samples maintained at a fixed temperature to induce phase separation; and cooling samples to achieve phase separation. Differences between “cloud point” and “phase separation point” are noted. Differences in performance between gasoline-alcohol mixtures and diesel fuel-alcohol mixtures are also examined and reported.