NOVELTY - System for blending butane into gasoline supply (260) which is also mixed with fixed ratio of ethanol, in amount that does not cause gasoline/ethanol mix to exceed at least one fixed volatility limit, comprises: supply of gasoline, ethanol standard and butane; analyzing system; information processing unit (IPU) for calculating from volatility ratio of butane that can be added to ethanol blended gasoline sample without passing fixed volatility requirement; and blending unit for blending butane from butane supply with gasoline from gasoline supply at or below butane ratio.    USE - The system is useful for blending butane into a gasoline supply which is also mixed with a fixed ratio of ethanol.    ADVANTAGE - The system blends butane into gasoline that is intended for ethanol blending, in a manner that maximizes the amount of butane that can be blended without exceeding or falling below (that is passing) pre-set volatility limits.    DETAILED DESCRIPTION - System for blending butane into a gasoline supply (260) which is also mixed with a fixed ratio of ethanol, in an amount that does not cause the gasoline/ethanol mix to exceed at least one fixed volatility limit which is vapor pressure, vapor liquid ration, T(10) (temperature at which 10% of the gasoline volatilizes) or T(50) (temperature at which 50% of the gasoline volatilizes), comprises: a supply of gasoline, ethanol standard and butane; an analyzing system for blending the gasoline sample with an ethanol standard at the fixed ratio to provide an ethanol-blended gasoline sample, and for measuring the volatility of the ethanol-blended gasoline sample; an information processing unit (IPU) for calculating from the volatility a ratio of butane that can be added to the ethanol blended gasoline sample without passing the fixed volatility requirement; and a blending unit for blending butane from the butane supply with gasoline from a gasoline supply at or below the butane ratio, where the gasoline supply varies over time in terms of content and volatility potential. An INDEPENDENT CLAIM is also included for blending a first volatility modifying agent (FVMA) into a petroleum supply which is also mixed with a fixed ratio of a second volatility modifying agent (SVMA), in an amount that does not cause the petroleum/SVMA mix to exceed at least one fixed volatility limit, comprising: (a) providing a supply of petroleum, the SVMA standard and the FVMA; (b) analyzing the volatility of a sample formed by mixing the petroleum and SVMA standard; (c) calculating from the volatility a ratio of FVMA that can be blended into the sample without causing the sample to pass the one or more fixed volatility limits; and (d) blending FVMA from the FVMA supply with petroleum from the petroleum supply at or below the ratio calculated in step (c), where the petroleum supply varies over time in terms of content and volatility potential.    DESCRIPTION OF DRAWING(S) - The figure shows a functional block diagram of a butane, ethanol and gasoline blending system.    Gasoline supply (260)    Sample selection station (270)    Static mixer (305)    Sample retention station (311)    Sample pump (313)