Ethanol and diesel oil mixtures are potential candidates as fuels which display advantages such as a reduced emission of particulates and the use of a renewable fuel (ethanol). However, their use is hampered because of limited miscibility, especially at lower temperatures. This problem can be overcome with the use of certain additives, which produce increased miscibility. This work investigates the role of these additives in this mixing process, with particular emphasis on assessing earlier propositions that microemulsions are formed in such mixtures. Results obtained from light-scattering measurements provide the first direct evidence for microemulsion formation both in diesel oil and ethanol and in synthetic diesel (a mixture of hydrocarbons that mimic diesel oil properties) and ethanol and additives. The effects of the additive type and concentration, temperature, and volume fraction of the ethanol on these microemulsion droplets' radii are presented, and their trends were found to follow those established for water-in-oil microemulsion systems.