TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Component: The hydrocarbon of component (a) is triptane. The component (b) comprises component (c) and (d). The component (c) which is more volatile has a lower boiling point than triptane and a Motor Octane greater than 88. It is butane or preferably iso-pentane. The component (d) has a boiling point higher than triptane but less than 170 degreesC and Motor Octane Number of at least 92. It is iso-octane. The component (b) as an oxygenate octane booster is an ether, preferably methyl or ethyl tertiary butyl ether.Preferred Property: The composition has Reid Vapor Pressure of 38-60 (preferably 38-49) kPa at 37.8 degreesC.Preferred Composition: The composition comprises 5-40 (preferably 5-25) vol.% of iso-pentane. It has up to 30 (preferably 5-30) vol.% 6-8C aromatic liquid hydrocarbon (preferably toluene). It has 40-90 (preferably 50-90) vol.% triptane, 1-3.5 vol.% butane and 10-35 vol.% iso-octane. It may contain 5-25 vol.% iso-pentane and 5-30 vol.% toluene. The volume ratio components (c)/(d) is 35-15:65-85. It has an isomerate fraction of 30-70% iso-pentane in which 6-44% of the composition is isomerate. The isooctane is present in a mixture of iso 7C and 8C hydrocarbons with small amount of iso 6C and 9C hydrocarbons. The mixture provides 8-55% of the total volume of the composition. EXAMPLE - An unleaded aviation gasoline blend was made by mixing 55 vol.% 2,2,3-trimethyl butane (99% purity) with 10 vol.% isomerate of 54.4% isopentane, 14.1% 2,2-dimethylbutane, 19.1% 2 and 3 methylpentanes and 5-10C hydrocarbons); 3 vol.% butane; 20 vol.% isooctane and 12 vol.% alkylate fraction of 51% isooctane, 21% trimethyl pentanes and 22% mixed isomeric hydrocarbons. The gasoline had 40.9 kPa and greater than 133 superchange performance number. A formulated unleaded aviation gasoline contained the above blend and 15 mg/L phenol antioxidant of 55% 2,4 dimethyl-6-tertiary butyl phenol, 15% 4 methyl-2,6-ditertiaty-butyl phenol and a balance of monomethyl and dimethyl-tertiary butyl phenols. The gasoline was tested for carbon dioxide and carbon monoxide emissions. Results showed that the unleaded aviation gasoline effectively reduced the emissions.