This work aims to clarify preferred chemical routes in an autothermal system by investigating individually two-carbon molecules containing the functional groups found in bio-oil. In part 2, conversion and selectivity to major compounds are compared in the presence and absence of α-Al 2O3 support and oxygen. Oxygen significantly increases conversion, even at feed rates insufficient to sustain autothermal operation. In general, below 450 °C, homogeneous reactions in the absence of oxygen are insignificant for all molecules; in the presence of oxygen, the reaction onset temperature is somewhat lower. The role of the α-Al2O 3 support appears to be a combination of improved heat transfer and the radical quenching ability of the foam structure. Acid catalysis appears minor compared to these other functions. With no O2 co-feed, acids, ethers, and aldehydes appear to be more stable than alcohols and esters. With O2 co-feed, acids and ethers may be less reactive in the gas phase than the other functionalities studied. Implications for autothermal systems are discussed.