Abstract: Formaldehyde, ketene, acetaldehyde, propanal and acetone in the gas phase of cigarette mainstream smoke were studied by synchrotron photoionization/time-of-flight mass spectrometry (TOF-MS). The mass-to-charge ratios (m/z) of these aldehydes and ketones can be obtained from photoionization mass spectra at different photon energies. Moreover, by tuning photon energies, the photoionization efficiency (PIE) curves of these compounds can be measured. Hence, structural identification can be fulfilled by comparing the measured PIE curves and ionization energies (IEs) with standard photoionization cross sections and IEs.