Oxalic acid is the single most abundant organic compound identified in ambient aerosols. In a previous study it was suggested that vehicular emissions were an important primary source of oxalic acid in an urban atmosphere. We here present size-segregated measurements of oxalic acid in a road-way tunnel and in an ambient urban environment in the Pearl River Delta Region, China, to demonstrate clearly that vehicular emissions are not a significant primary source for oxalic acid in urban aerosols. Evidence in support of this assertion includes: (1) no enhancement in oxalate levels in the tunnel aerosol was seen in comparison with the oxalate levels in the ambient environment, (2) oxalate in the tunnel samples bore little resemblance in its size distribution to that of elemental carbon and had no correlation with elemental carbon, and (3) the ratio of oxalate/elemental carbon in the tunnel samples was more than 15 times lower than that in the ambient samples.