Concentrations of dissolved and particulate 2 3 0 Th and 2 3 2 Th in surface seawater decreased from near-shore to off-shore in the Gulf of Mexico, demonstrating terrestrial input to the surface waters. Vertical profiles indicate that lateral transport, as well as surface inputs and bottom sediment resuspension, could be an important source of 2 3 0 Th and 2 3 2 Th in ocean margin regions. Sea water samples collected off Cape Hatteras contained significantly higher dissolved and particulate 2 3 0 Th and 2 3 2 Th concentrations. Likely, the distributions of 2 3 0 Th in seawater off Cape Hatteras are, to a large extent, controlled by terrestrial particles. In contrast, in-situ production (for 2 3 0 Th) and scavenging are the major processes regulating the concentrations of dissolved 2 3 0 Th and 2 3 2 Th in the open Gulf of Mexico. The overall residence time of 2 3 0 Th in the water column at a slope station in the Gulf of Mexico is calculated to be about 5 yr.