Ethanol and acetaldehyde emissions from a direct ignition spark ignition were measured using mass spectrometry. Previous methods focused on eliminating or minimizing interference from exhaust species with identical atomic mass and fragment ions created in ionization process. This paper describes a new technique which exploits the fragment ions from ethanol and acetaldehyde. A survey of mass spectra of all major species of exhaust gas was conducted. It was found that ethanol contributes most ions in mass number 31 and that no other gas species produces ions at this mass number. Acetaldehyde detection suffers more interference. Nevertheless, it was estimated that detection at mass number 43 is possible with 10% error from 2-methylbutane.This new technique was validated in an engine experiment. By running the engine with pure gasoline and E85, the validity of the technique can be checked. Two conditions were investigated: idling (1200 rpm, 1.5 bar NIMEP, retarded ignition timing) and medium load (1500 rpm, 3.8 bar NIMEP, MBT ignition timing). The results from both conditions confirmed that ions were only detected when E85 was used. Furthermore, the measured ethanol concentrations agree with results obtained using gas chromatography. However, acetaldehyde was overestimated greatly. It was possibly caused by ions with atomic mass 44 being miscounted or by the fact that the interference from 2-methybutane was much bigger than calculated. Further investigation is required.