The relationship between the physical and chemical properties and performance of gasolines and diesel fuel are discussed. For gasolines, emphasis is placed on the effects of oxygenates when used as gasoline blending components. With the phase-out of leaded gasoline, the use of oxygenated blending agents is increasing in order to meet the octane demands for unleaded gasolines. Octane and volatility are the two most important quality aspects of gasolines. Oxygenates, particularly methanol and ethanol, have definite effects on these properties. The correlation between gasoline composition and its effect on other areas, e.g., emissions, water tolerance and elastomers, are also shown.