Long-range transport of sulfur dioxide (SO2) from east Asia to the central North Pacific troposphere was observed on transit flights during the NASA Transport and Chemical Evolution over the Pacific mission. A series of SO2-enhanced layers above the boundary layer was observed during these flights. The significant features included enhanced SO2 layers associated with low water vapor and low turbulence that were usually dynamically isolated from the marine boundary layer. This study shows that atmospheric dynamics were very important in determining the SO2 distributions in the central Pacific during March and April 2001. Trajectory studies revealed that SO2-enhanced layers could be connected to both volcanic and anthropogenic sources in east Asia. These trajectory studies also showed that the air parcels usually were lifted 2 km above the source regions and then progressed to the east in the midlatitudes (30°–60°N). The air parcels arrived in the central Pacific within 2–3 days. Sulfur dioxide transported at altitudes of 2–4 km dominated the SO2 distribution in the central Pacific. A comparison of SO2 observations and results of chemical transport models indicated that SO2 was removed primarily by cloud processes. Therefore, in the absence of cloud, SO2 can be transported long distances if the trajectory is decoupled from the boundary layer. Another important observation was that the Miyake-jima volcano made a major contribution to the SO2 concentrations in the central Pacific troposphere during March and April 2001. At times, the volcanic SO2 had more influence in the central Pacific than the six largest anthropogenic SO2 source regions in east Asia.