Diesel vehicle exhaust is an important source of carbonaceous aerosols, especially in developing countries, like China. Driving condition impacts diesel vehicle emissions, yet its influence needs further understanding especially on secondary organic aerosol (SOA) formation. In this study tailpipe exhaust from an in-use light duty diesel vehicle at idling and driving speeds of 20 and 40 km h(-1) was introduced respectively into a 30 m(-3) indoor smog chamber to investigate primary emissions and SOA formation during photo-oxidation. The emission factors of SO2 at 20 and 40 km h(-1) were higher than those at idling, whereas the emission factors of aromatic hydrocarbons (AHs), polycyclic aromatic hydrocarbons (PAHs) and oxygenated volatile organic compounds (OVOCs) decreased when driving speeds increased. The emission factors of black carbon (BC) and primary organic aerosol (POA) at idling were comparable to those at 20 and 40 km h(-1). The SOA production factors were 0.41 +/- 0.09 g kg-fuel(-1) at idling, approximately 2.5 times as high as those at 20 km h(-1) (0.16 +/- 0.09 g kg-fuel(-1)) or 40 km h(-1 )(0.17 +/- 0.09 g kg-fuel(-1)). Total carbonaceous aerosols, including BC, POA and SOA, from diesel vehicles at 20 and 40 km h(-1) were 60-75% of those at idling, due largely to a reduction in SOA production. Measured AHs and PAHs altogether were estimated to explain