The invention relates to systems and methods for modifying the volatility of petroleum prior to ethanol addition. The methods can include (a) providing (i) a supply of gasoline, (ii) an ethanol standard, and (iii) a supply of butane; (b) analyzing the volatility of a sample formed by mixing the gasoline and ethanol standard; (c) calculating from the volatility a ratio of butane that can be blended into the sample without causing the sample to pass the one or more fixed volatility limits; and (d) blending butane from the butane supply with gasoline from the gasoline supply at or below the ratio calculated in step (c).