This paper describes experiments conducted to determine the regulated emissions, ozone-forming potentials, specific reactivities, and reactivity adjustment factors for eight butane and propane alternative fuel blends run on a light-duty vehicle, emission certified to be a California transitional low emission vehicle (TLEV) and converted to operate on liquefied petroleum gas (LPG). Duplicate EPA FTP emission tests were conducted with each fuel. Hydrocarbon speciation was utilized to determine reactivity-adjusted non-methane organic gases (NMOG) emissions for one test on each fuel. Results showed that all eight fuels could allow the converted vehicle to pass California ultra-low emission vehicle (ULEV) NMOG and oxides of nitrogen (NOx) standards. Six of the eight fuels could allow the vehicle to pass ULEV carbon monoxide (CO) standards.BUTANE has been an important gasoline blending component for many years. With a road octane number of 92 and a high blending vapor pressure, butane has been used to upgrade the octane of gasoline blends and to aid in wintertime cold starting. Due to reformulated gasoline requirements for lower fuel vapor pressure, industry has had to remove increasing amounts of butane from the gasoline pool. Paradoxically, butane is one of the cleanest burning components of gasoline. This increasing inability to use butane in motor fuel represents both an economic and environmental waste.Other than reformulated gasoline and clean diesel, liquefied petroleum gas (LPG) is the most widely used alternative fuel in the United States. The primary constituent of LPG used for this purpose is propane. Propane has many attributes which make it an attractive vehicular fuel. Since it is a single relatively simple species, engines and aftertreatment systems can be designed to burn it cleanly. It also reacts very slowly in the atmosphere so it tends to form less ground level ozone in cities. Propane is also stored onboard as a liquid at ambient temperature in a pressurized system; therefore, by definition, it produces no evaporative emissions. As a liquid, it has a good volumetric energy content so vehicle range is improved over many of the other alternative fuels. The road octane number of propane is greater than 100 so vehicle power can be optimized in vehicles equipped to take advantage of this property. There is also a wide distribution system for propane so vehicle refueling is performed more readily than with most of the alternatives.Importantly, however, barriers exist which may inhibit the growth of propane as a motor fuel. There are other developed, mature markets for propane such as its use as a heating fuel or as a chemical feedstock. Propane is produced as a side product from natural gas production or petroleum refinery processing. Additional demand could create shortages and/or price increases, because it is unlikely that either of these two sources would be ramped up solely to produce more propane. Although vehicle range is good, it remains three-fourths that of conventional gasoline vehicles. These constraints have limited LPG vehicle implementation.A thoughtful consideration of the use of butane as a motor fuel shows that it should have many of the positive attributes of propane. Butane is also a single, relatively simple chemical species so theoretically engines and exhaust catalysts could be optimized for low emissions with its use. It also has relatively low reactivity in the atmosphere and like propane, would be stored on-board as a pressurized liquid. Consequently, a butane-fueled vehicle would also have no evaporative emissions, and could also be classified as an inherently low emissions vehicle (ILEV). Butane has a greater volumetric energy content than propane so vehicle range could be extended over that of propane. Since butane storage pressure is lower than that of propane, there is an opportunity to use the same equipment for delivery and refueling of butane as is currently used for propane.Butane tends to be in slight oversupply on a yearly basis in the U.S. and this excess should grow larger as gasoline volatility is reduced. The imbalance is projected to equal as much as 100,000 barrels per day this year and increase by 20 percent by the year 2000.(1)* Although butane is used as a chemical feedstock, it has not been used extensively as a heating fuel as has propane. Like propane, butane production is associated with natural gas production and petroleum refining. However, butane production is more easily swung between gasoline and neat product than propane. This could allow butane supply to more easily balance demand than propane.