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Molecular compositions and stable carbon isotope ratios (PC) of n-alkanes and fatty acids (FA) were investigated in urban aerosols from Beijing, China. Seasonal trends for n-alkanes showed a higher carbon preference index (CPI) and C-max at C-29 in spring indicating the enhanced emission of higher plant waxes. Lower CPIs and C-max at C-23 were found in winter that should be associated with fossil fuel combustion. C-13-depletion in C-29 n-alkane was observed during an extreme rainfall event, during which the inflow of air masses were originated from lower latitude regions. Higher concentrations of C-13-enriched C-23 n-alkane in winter together with the air mass trajectories suggested that the Beijing aerosols were affected by the air masses transported from the southern regions. Fatty acids were abundant in fall and showed a bimodal pattern with C(max )at C-16(:0) and C-18(:0). Higher C-24(:0)/FA ratios with C-13-enrichment occurred as influenced by southerly air masses, and lower C-24(:0)/FA ratios with C-13-depletion was observed with northerly air masses in fall and winter. These results suggested that C-24(:0) fatty acid was more abundantly transported from the southwestern air masses and may be a powerful tool to distinguish polluted aerosol source regions. Moreover, the delta C-13 values of C-24(:0) fatty acid showed a positive correlation with levoglucosan, implying that the biomass-burning process may contribute C-13-enriched C-2(4:0) fatty acid.