A measurement campaign was conducted at a semi-urban site located in the suburbs of the city of Beirut (Lebanon) during summertime (2–18 July 2011). The molecular composition of organic PM2.5 was investigated following a chemical derivatization gas chromatography/mass spectrometry technique. Accordingly, several classes of compounds represented by 18 individual organic tracers were determined. These tracers include levoglucosan, a tracer for biomass combustion, dicarboxylic acids, and several tracers for the photo-oxidation of isoprene, α-pinene and β-caryophyllene. The sum of the mean concentrations of the isoprene oxidation products was 4 ng/m3, that of α-pinene was 124 ng/m3 and that of β-caryophyllene was 11 ng/m3. For other tracers of organic aerosols, the highest concentrations were obtained for carboxylic acids with an average value of 939 ng/m3. An average value of 49 ng/m3 was obtained for levoglucosan. Organic and elemental carbon concentrations were measured by a thermo-optical analyzer. Average values were 5.6 and 1.8 μg/m3, respectively. A reconstruction of organic PM2.5 composition suggests that cooking, fossil-fuel combustion, biomass burning, sesquiterpenes, monoterpenes, and isoprene contribute on average about 27 ± 13, 16 ± 7, 5 ± 3, 26 ± 5, 26 ± 13 and < 1 ± 0.3% of PM2.5 organic carbon, respectively.