The oxidation of 1-butene and n-butane in air at elevated pressure was investigated in a high pressure chemical flow reactor. Results are presented for pressures of 3, 6, and 10 atm, temperatures near 900K, and lean equivalence ratio. Gas samples were analyzed using gas chromatography with aldehydes sampled using a dinitrophenylhydrazine/acetonitrile procedure employing gas chromatography/mass spectrometry analysis. Major common products observed include CO, CH2O, C2H4, C3H6, and CO2. Additional major products included 1,3-C4H6 for 1-butene and 1-C4H8 for n-butane. Fuel conversion was increased with increased pressure, temperature, and equivalence ratio with 1-butene more reactive than n-butane. Large levels of lower molecular weight carbonyls resulted from 1-butene whereas significant amounts of conjugate and lower molecular weight alkenes resulted from n-butane. Trends in product distributions with increasing pressure were successfully accounted for by current autoignition theories. Paraffin autoignition chemistry is characterized by hydrogen abstraction from the fuel, followed by molecular oxygen addition to alkyl radicals, with several possible alkylperoxy routes determining final product distribution. Olefin autoignition chemistry involves radical addition to the double bond directly, ultimately yielding carbonyls and cyclic ethers.