Real-world vehicle emissions of carbonyls compounds (CC) were determined at the Rebouças Tunnel, Rio de Janeiro (Brazil). The tunnel is a two-bore tunnel with three lanes in each direction and has a length of 2840 m, divided in two sections. On average, approximately 5,000 vehicles (95% light duty) were passing the tunnel per hour. Sixteen samples were simultaneously collected 500 and 1500 m inside from the entrance. The main CC were formaldehyde, acetaldehyde, acetone, propionaldehyde and benzaldehyde. A total of 16 samples were collected in each point in 8 different days. In 5 days, CO and CO2 were also monitored during the sampling period. Emission factors were calculated. Using the Pierson method, emission factors were 7.5 ± 2.6 and 13.8 ± 5.7 mg km-1 for formaldehyde and acetaldehyde, respectively. Using the fuel consumption method, values were 6.3 ± 2.1 and 11.8 ± 3.9 mg km-1 for the same compounds. The differences between both methods were lower than 20% and may be considered acceptable considering all the approximations in the calculations. Ozone forming potentials (OFP) were also estimated as 90.4 ± 37.3 and 70.7 ± 24.6 mg km-1 for formaldehyde and acetaldehyde, respectively.