Glyoxal and methylglyoxal, the two smallest yet most abundant dicarbonyls, play vital roles in forming secondary organic aerosols (SOA) in the ambient air. The direct sources for glyoxal and methylglyoxal from vehicles are still unclear because of only a few investigations in the USA. Here we carried out tests in the Zhujiang tunnel in urban Guangzhou in south China to obtain emission factors (EFs) of glyoxal and methylglyoxal for on-road vehicles. Measured EFs for glyoxal and methylglyoxal averaged 1.18 ± 0.43 and 0.52 ± 0.26 mg km−1 veh−1, and were about 6.6 and 2.3 times those measured in the Tuscarora Mountain Tunnel in 1999 (Grosjean et al., 2001), respectively. Multiple linear regressions further resolved glyoxal EFs of 1.64 ± 1.03, 0.10 ± 3.49 and 0.58 ± 2.37 mg km−1 and methylglyoxal EFs of 0.17 ± 0.33, 1.68 ± 1.20 and 0.70 ± 0.66 mg km−1, respectively, for gasoline, diesel and liquefied petroleum gas (LPG) vehicles. The fuel-based EFs for glyoxal and methylglyoxal were estimated to be 28.1 and 2.9 mg kg−1 for gasoline vehicles, and 1.5 and 26.3 mg kg−1 for diesel vehicles, respectively. Based on available SOA yields, SOA formed from vehicle-emitted glyoxal and methylglyoxal could attain 25–50% of that formed from vehicle-emitted toluene. With the EFs from this study, the vehicle emission of the two dicarbonyls in China and in the world were roughly estimated. Either the CO-tracer-based or the fuel-based global estimates are below 0.1 Tg a−1 and therefore vehicle emission could be negligible in their global total sources, yet they might play vital roles in urban areas in forming SOA, particularly in the early chemical evolution of vehicle exhausts in the ambient. •Road vehicle emission factors for glyoxal and methylglyoxal were obtained by tunnel tests.•Vehicle glyoxal and methylglyoxal EFs in China were compared with those in the US.•Mileage-based and fuel-based EFs were resolved by multiple linear regressions.•Vehicle emissions of glyoxal and methylglyoxal in China and in the world were estimated.