Stable carbon isotope ratio ( δ 13C) data can provide important information regarding the sources and the processing of atmospheric organic carbon species. Formic, acetic and oxalic acid were collected from Zurich city in August–September 2002 and March 2003 in the gas and aerosol phase, and the corresponding δ 13C analysis was performed using a wet oxidation method followed by isotope ratio mass spectrometry. In August, the δ 13C values of gas phase formic acid showed a significant correlation with ozone (coefficient of determination ( r 2) = 0.63) due to the kinetic isotope effect (KIE). This indicates the presence of secondary sources (i.e. production of organic acids in the atmosphere) in addition to direct emission. In March, both gaseous formic and acetic acid exhibited similar δ 13C values and did not show any correlation with ozone, indicating a predominantly primary origin. Even though oxalic acid is mainly produced by secondary processes, the δ 13C value of particulate oxalic acid was not depleted and did not show any correlation with ozone, which may be due to the enrichment of 13C during the gas - aerosol partitioning. The concentrations and δ 13C values of the different aerosol fractions (water soluble organic carbon, water insoluble organic carbon, carbonate and black carbon) collected during the same period were also determined. Water soluble organic carbon (WSOC) contributed about 60% to the total carbon and was enriched in 13C compared to other fractions indicating a possible effect of gas - aerosol partitioning on δ 13C of carbonaceous aerosols. The carbonate fraction in general was very low (3% of the total carbon).