The oxidation of ethanol blended with gasoline surrogates, a mixture of iso-octane and n-heptane, was studied in a flow reactor to evaluate the influence of ethanol content and oxygen concentration on the oxidation behaviors of reactants and the emissions of methanol, formaldehyde, and acetaldehyde. The blend with 10% ethanol content accelerated the emergence of the negative temperature coefficient region of iso-octane and n-heptane. The emissions of formaldehyde and acetaldehyde increased with the increasing ethanol content. Increasing the oxygen concentration from 10% to 21% led to increased emissions of methanol, acetaldehyde, and formaldehyde. With the oxygen concentration from 21% to 30%, methanol and acetaldehyde emissions decreased. The sources of aldehydes were explored and were found to be formed from different sources in different temperature regions. At low temperature, acetaldehyde was partially derived from ethanol oxidation and partially from n-heptane oxidation; formaldehyde emission was mainly contributed to by ethanol oxidation. At high temperature, acetaldehyde was mainly produced from ethanol oxidation; formaldehyde was partially formed from iso-octane oxidation and partially from the consumption of methanol.