With increasing use of ethanol in automotive fuel in recent years, which can be made from renewable feedstocks, the chemical composition of gasoline is changed. The compositional change results in many changes in fuel properties. One key property is the octane rating of gasoline. Market data has shown the shifts of octane rating (antiknock index or AKI) upward due to more penetration of E10 gasoline in the US market. However, the increase in research octane is more pronounced as compared to motor octane, therefore the increase in octane sensitivity in gasoline. Refineries have used the change in octane due to ethanol contribution by sending so called sub-grade gasoline to terminals expecting the final blend after mixing with ethanol to meet the market requirement in octane. Thus the octane rating of the final blend will largely depend on the sub-grade gasoline composition and ethanol. This paper presents a general model for the prediction of octane values based on fuel composition and properties of gasoline with different levels of ethanol contents. Thirty-six gasoline samples with different ethanol content are examined using gas chromatography for detail compositional determination. These samples are market representative and analyzed for research octane (RON) and motor octane (MON) and other fuel properties as fuel volatility and density. Correlation of detail chemical compounds to the response of RON and MON are performed. Contributions of chemicals or chemical interactions to RON/MON rating are presented and discussed.