The gas and particulate phase of semi-volatile organic compounds (six samples) and the composition of organic aerosol as a function of particle size (six samples) were studied in two public buildings in Greece. The objectives of this study were: i) to chemically characterize the organic content of indoor gas and particulate phases; ii) to classify indoor organic aerosol constituents as a function of particle size; and iii) to reconcile the sources of organic compounds indoors. Non-polar, semi-polar, polar, and acidic compounds were identified in both gas and particulate phases by using gas chromatography/mass spectrometry (GC/ MS). Branched iso- and anteiso-alkanes were used to trace environmental tobacco smoke (ETS) concurrently with other compounds. Polycyclic aromatic hydrocarbons (PAHs) in the gas phase gave a pattern more characteristic to ETS than the corresponding pattern in the particulate phase. The chemical composition observed for n-alkanols and n-alkanoic acids in both gas and particulate phases indicated ETS as one of their main sources indoors. PAHs and n-alkanols were evenly associated between fine and coarse particles and their corresponding total mean mass median aerodynamic diameter (MMAD) was 1.27 and 1.38 μm respectively, indicating a mixed origin. Conversely, the MMAD of n-alkanes, unresolved complex mixture (UCM), iso- and anteiso-alkanes and free fatty acids varied from 0.30 to 0.62 μm denoting a stronger association with indoor sources.