ABSTRACT This study is aimed to investigate the transport of air pollutants and traffic flow in Chungcheng and Cross-Harbor Tunnels in Kaohsiung City. The work includes road measurements and three-dimensional numerical modeling. This article reports the first part of the work concerning measurement results. The items on road-side investigation include traffic flow rate traffic, speed, travel time and vehicle catergory. The air pollutants being sampled and analyzed in the tunnels are carbon monoxide (CO), sulfur dioxide (SO2), nitrogen oxides (NOX) and total hydrocarbon (THC). The road-side investigations show that the major vehicles in Chungcheng Tunnel are passenger cars, the next are small trucks, and the minor are large trucks. The peak hours are at 10:30 ¡V 11:30 in the morning and 15:30 ¡V 17:30 at afternoon with average traffic flow rate in 250 ¡V 350 No./hr. The major vehicles in Cross-Harbor Tunnel are motorcycles, the next are passenger cars, large trucks and small trucks. Traffic flow rate at morning peak hours of 7:00 ¡V 8:00 are 1,800 and 1,000 No./hr for motorcycles and passenger cars, respectively, and are below 800 No./hr for the large and small trucks. Results of air pollutants analysis show that CO in Cross-Harbor Tunnel is about as three times as that of Chungcheng Tunnel due to high motorcycle flow rate in Cross-Harbor Tunnel. The maximum concentration of NOX was about 500 ppb for Cross-Harbor Tunnel and was about 1,000 ppb for Chungcheng Tunnel during the study period, suggesting that cars and trucks are the major emission sources of NOX. The transport of air flow and gaseous pollutants downstream (or to the tunnel exit) is mainly done by tunnel fans and piston effect of moving vehicles in order to decrease pollutants concentrations inside the tunnel. The mechanism is affected by fan number and flow rate, traffic flow rate, traffic speed and net tunnel aspect ratio. Results in two tunnels show that CO and NOX both increase with increasing downstream distance, but are not always so for SO2 and THC. Whether other mechanisms such as absorption of SO2 and THC on tunnel walls or sink and absorption on tunnel floor due to being heavier than air affect distributions of SO2 and THC remains further investigations.